I've just read an answer to this inwhich the respondent objects that there is no more vacuumy a vaccum than total absence of matter, and that plots of the speed of light versus density extrapolated cross the zero density axis at the received value of $c$; but I presumed that the OP, in referring to beyond a vacuum meant beyond in the sense of virtual particles & vaccum energy & all that. If so ... it's conceivable that it would ... and yet that its doing so have no bearing on the possibility of any signal or substance travelling faster than this value. This kind of situation already exists in connection with radio waves & the ionosphere, in that the ionosphere technically has a refractive index to radio waves (in a certain frequency-band) less than unity. In such a medium, the speed of electromagnetic waves is greater than $c$ ... but - even theoretically - this superluminal speed of the underlying wave does not admit of any thing, in any sense in which "thing" can reasonably be said to have any meaning, being conveyed at superluminal speed. This is because any wave-packet composed of waves having any spread of wavelengths, howsoever slight that spread might be - even in the limit as the spread tends to zero, travels at the group velocity, which is always less than $c$ (or equal in the limit as the spread tends to zero). There is only meaning in the notion of conveying any substantial thing insofar as the wave varies - the notion of varying here comprising any stopping and-or starting of the signal. The wave speed, which has the superluminal size, only has this velocity insofar as it has infinite temporal extension both pastwards and futurewards, with which extension it is of a piece that there exists zero grounds for even so much as the notion of its speed being in any way immediately manifest - which is to say manifest actually as a speed . And yet it is manifest mediately, through the the bending of radio waves back earthwards at the boundary at which the ionosphere begins.

Basically, you find that whatever happens even in the realm of pure theory, the notion of any substantial or in any way discernible thing being conveyed at superluminal speed is always foiled - by reason of that very discernibility. And I am proposing it here that even if the answer to the OP's question could be reasonably maintained to be "yes", there would be a similar foiling of that notion of a piece with whatever reasoning it is by which it were so maintained.

I've just read an answer to this inwhich the respondent objects that there is no more vacuumy a vaccum than total absence of matter, and that plots of the speed of light versus density extrapolated cross the zero density axis at the received value of $c$; but I presumed that the OP, in referring to beyond a vacuum meant beyond in the sense of virtual particles & vaccum energy & all that. If so ... it's conceivable that it would ... and yet that its doing so have no bearing on the possibility of any signal or substance travelling faster than this value. This kind of situation already exists in connection with radio waves & the ionosphere, in that the ionosphere technically has a refractive index to radio waves (in a certain frequency-band) less than unity. In such a medium, the speed of electromagnetic waves is greater than $c$ ... but - even theoretically - this superluminal speed of the underlying wave does not admit of any thing, in any sense in which "thing" can reasonably be said to have any meaning, being conveyed at superluminal speed. This is because any wave-packet composed of waves having any spread of wavelengths, howsoever slight that spread might be - even in the limit as the spread tends to zero, travels at the group velocity, which is always less than $c$. There is only meaning in the notion of conveying any substantial thing insofar as the wave varies - the notion of varying here comprising any stopping and-or starting of the signal. The wave speed, which has the superluminal size, only has this velocity insofar as it has infinite temporal extension both pastwards and futurewards, with which extension it is of a piece that there exists zero grounds for even so much as the notion of its speed being in any way immediately manifest - which is to say manifest actually as a speed . And yet it is manifest mediately, through the the bending of radio waves back earthwards at the boundary at which the ionosphere begins.

Basically, you find that whatever happens even in the realm of pure theory, the notion of any substantial or in any way discernible thing being conveyed at superluminal speed is always foiled - by reason of that very discernibility. And I am proposing it here that even if the answer to the OP's question could be reasonably maintained to be "yes", there would be a similar foiling of that notion of a piece with whatever reasoning it is by which it were so maintained.

I've just read an answer to this inwhich the respondent objects that there is no more vacuumy a vaccum than total absence of matter, and that plots of the speed of light versus density extrapolated cross the zero density axis at the received value of $c$; but I presumed that OP, in referring to beyond a vacuum meant beyond in the sense of virtual particles & vaccum energy & all that. If so ... it's conceivable that it would ... and yet that its doing so have no bearing on the possibility of any signal or substsnce travelling faster than this value. This kind of situation already exists in connection with radio waves & the ionosphere, in that the ionosphere technically has a refractive index to radio waves (in a certain frequency-band) less than unity. In such a medium, the speed of electromagnetic waves is greater than $c$ ... but - even theoretically - this superluminal speed of the underlying wave does not admit of any thing, in any sense in which "thing" can reasonably be said to have any meaning, being conveyed at superluminal speed. This is because any wave-packet composed of waves having any spread of wavelengths, howsoever slight that spread might be - even in the limit as the spread tends to zero, travels at the group velocity, which is always less than $c$ (or equal in the limit as the spread tends to zero). There is only meaning in the notion of conveying any substantial thing insofar as the wave varies - the notion of varying here comprising any stopping and-or starting of the signal. The wave speed, which has the superluminal size, only has this velocity insofar as it has infinite temporal extension both pastwards and futurewards, with which extension it is of a piece that there exists zero grounds for even so much as the notion of its speed being in any way immediately manifest - which is to say manifest actually as a speed . And yet it is manifest mediately, through the the bending of radio waves back earthwards at the boundary at which the ionosphere begins.

Basically, you find that whatever happens even in the realm of pure theory, the notion of any substantial or in any way discernible thing being conveyed at superluminal speed is always foiled - by reason of that very discernibility.

I've just read an answer to this inwhich the respondent objects that there is no more vacuumy a vaccum than total absence of matter, and that plots of the speed of light versus density extrapolated cross the zero density axis at the received value of $c$; but I presumed that the OP, in referring to beyond a vacuum meant beyond in the sense of virtual particles & vaccum energy & all that. If so ... it's conceivable that it would ... and yet that its doing so have no bearing on the possibility of any signal or substance travelling faster than this value. This kind of situation already exists in connection with radio waves & the ionosphere, in that the ionosphere technically has a refractive index to radio waves (in a certain frequency-band) less than unity. In such a medium, the speed of electromagnetic waves is greater than $c$ ... but - even theoretically - this superluminal speed of the underlying wave does not admit of any thing, in any sense in which "thing" can reasonably be said to have any meaning, being conveyed at superluminal speed. This is because any wave-packet composed of waves having any spread of wavelengths, howsoever slight that spread might be - even in the limit as the spread tends to zero, travels at the group velocity, which is always less than $c$ (or equal in the limit as the spread tends to zero). There is only meaning in the notion of conveying any substantial thing insofar as the wave varies - the notion of varying here comprising any stopping and-or starting of the signal. The wave speed, which has the superluminal size, only has this velocity insofar as it has infinite temporal extension both pastwards and futurewards, with which extension it is of a piece that there exists zero grounds for even so much as the notion of its speed being in any way immediately manifest - which is to say manifest actually as a speed . And yet it is manifest mediately, through the the bending of radio waves back earthwards at the boundary at which the ionosphere begins.

Basically, you find that whatever happens even in the realm of pure theory, the notion of any substantial or in any way discernible thing being conveyed at superluminal speed is always foiled - by reason of that very discernibility. And I am proposing it here that even if the answer to the OP's question could be reasonably maintained to be "yes", there would be a similar foiling of that notion of a piece with whatever reasoning it is by which it were so maintained.