A vampire itself would have the useful ability to wear a pair of glasses with one lens mirrored on the inside, and thus be able to see people sneaking up behind it with stakes. Unfortunately, a vampire so equipped would no longer be entirely invisible in mirrors – there'd be a mirrored lens hovering in mid-air.

But as we can’t see a vampire’s clothes in the mirror, why should we be able to see a vampire’s glasses in the mirror?

Hmm. Well, it is possible that by wearing clothes and glasses a vampire can temporarily lend them the vampire-nature. But in that case, the mirrored glasses are even less useful: once the glasses become vampiric, they reflect S-photons, so the vampire can't use them to see behind it any more – it'd just see a closeup of its own left eyeball.

It also means that your use of a reflective vampire as a vampire shaving mirror (assuming vampires need to shave) is redundant, as it could be replaced by a wearable mirror.

Possibly, although once a mirror is large enough and far enough away from the vampire to be convenient to shave in, I think you're starting to stretch the definition of "wearing". I don't think just being strapped to the vampire's body can be a sufficient condition, otherwise a vampire in a car with its seat belt on would render the whole car reflectionless, and one strapped to the ground would do the same to the planet! :-)

But this is why in Anne Rice novels the vampires have to live in pairs, so that one can shave in the other's mirrored glasses.

Unless it's something like decoherence. When you consider the vampire+lenses as a whole, it acts as a vampire. But when you zoom in, they can also be considered separately. But that's just a guess, it's probably optically inconsistent.