If we assume that Vampires can go into a kind of torpor, I think that gets around the huge numbers of human prey they might need otherwise.
Well, sure! But, the origional arguement didn't mention this. A torpor is just fine.

The only problem is, you need to make sure there's someone to protect you, while you're in stasis.
I strongly suspect that those Vampires who do spawn more like themselves will vastly outnumber those few Vampires who don't.
Eh? I don't follow.
I didn't think he had to do it all in one night.
The origional arguement on this was that he did it in one night. Even if he Didn't do it in one night, I don't see him killing Half the villlage, then going back the next day and not expecting people to be Prepared (Even your commoner can create a wooden holy symbol of some good god). True, he Could bury them the next night.
Why don't old folks make decent Vampire Spawn?
From a purely rules groundpoint, bad Str and Dex. Those old bodies aren't used to anything agile or strong, and therefore their hitting and damage would be worse off, as would their dex. So we're looking at, from a commoner, a 7-9 Str and Dex. Even with a vampire spawn template applied, this doesn't strike me as very good.
From a gaming aspect, I'm not really going to be That fearful of grampa if he was a vampire. Sure, that'd be a sneaky surprise, but from a combat perspective, I think the spawn would be weaker then your average one. Let's put it like this: If you were recruting an army, would you include old people, or would you look for those around 16-30 in years?