They can only suck blood from units that have blood, so not undead or plants or golems etc. They can suck from most units, so your problem was likely that you were trying to make them suck from a unit that doesn't have blood. Another possible cause of the problem is that they will only replace units up to however many they started the combat with. So if you used sacrifice to increase the stack, that would prevent bloodsucking.
As for your other problem, I don't quite understand. You currently have ancient vampire bats, and you want to hire more? Well, you will have to find more bat form ones, because the game considers them separate units. No use of an ability in combat will change what the base unit is. Just like when you make your vampires turn into bats, they are just technically vampires in bat form, your bats just become bats in vampire form.
A word of warning: if you have both kinds of ancient vampires (both bat and man) in your army during a combat, and you make either stack change form, you will suddenly have too many of whichever type, and they will rebel against you.
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