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.