Actually, you can't do that for vacuum, for what amount of volume do you choose? Vacuum energy would obviously be proportional to volume, so your T would be proportional to the volume chosen. It's not meaningful. The k_b*T formula applies for each particle (with a 3/2 factor, usually), so you can never get T from the energy contained in a volume unless you know how many particles are in that volume, on average at least. So even early in the Big Bang, there would be particles, they just would be blinking in and out of existence. And no theory of the Big Bang can say that T was ever infinite, that's completely outside any physics we can make sensible statements about. Just cleaning up the terminology a bit...