There have been reports of physicist Miles Padgett at the university of Glasgow that the speed of photons in a vacuum can be made to vary slightly by changing the structure of a photon. Is this true? What do you need to do to the structure of a photon to change its velocity? Won't this have implications for the Standard Model of particle physics?
The answer to your question is that in your case it is group velocity and phase velocity.
The group velocity and phase velocity can be slower then c (or faster then c), when measured locally in vacuum.
Noting that c/n = vp, indicates that the group speed is equal to the phase speed only when the refractive index is a constant dn/dk = 0, and in this case the phase speed and group speed are independent of frequency, ω/k=dω/dk=c/n. Otherwise, both the phase velocity and the group velocity vary with frequency, and the medium is called dispersive; the relation ω=ω(k) is known as the dispersion relation of the medium. The phase velocity of electromagnetic radiation may – under certain circumstances (for example anomalous dispersion) – exceed the speed of light in a vacuum, but this does not indicate any superluminal information or energy transfer. It was theoretically described by physicists such as Arnold Sommerfeld and Léon Brillouin. See dispersion for a full discussion of wave velocities.
Now if you are talking about this:
Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.
Then this is the answer to your question.
Try to not imagine a photon as a little ball of light traveling through space like a bullet. A photon is a minimal excitation of the EM field. A photon has no velocity and no structure.