It’s just an idle thought, really, but what if vampires (in D&D in particular) actually needed intelligent prey?
They don’t chase them because they’re into gorgeous women in diaphanous nightgowns (though, honestly, that’s a good enough reason for me), but because their own intellect and ability to hold back their bestial hungers depend on the native intelligence of the creatures they draw blood from?
I mean, if I were a vampire that wanted to go uncaught, I’d be picking off animals in the night, or even get a job in a butcher shop and take my work home with me. It’s possible that humans might taste better, but if it means not getting staked through the heart I’d be willing to change my diet.
Hell, I stopped eating wheat and traded grain-fed beef for grass-fed (which took some getting used to, I have to admit) because it’s healthier for me, I can’t see why a vampire might not switch from human to beef or pork blood if it means hiding his nature from those around him.
Okay, yeah, might be a little challenging finding an abattoir with a graveyard shift, but it may be possible.
It might even explain differences in vampires. The more bestial ones, of lesser intellect and capability (but possibly greater physical power), might be the ones with a restricted diet. They are constrained to eating from animals rather than fully-aware creatures, and so take on some of the more bestial aspects — greater strength, endurance, agility, senses, whatever. It may be that the ‘vampire spawn’ described in the RSRD aren’t truly less than the ‘full vampire’ (template), they merely don’t get to ‘tap the good stuff’ that would let them increase their intellect and power.