I can't say I know enough about the naga to give a yes/no about them being vampires. What matters is if Aarem is willing to accept their souls/has a soul to give, so assuming nagas are not soulless beings they're free to give it to whomever, be it Aarem or their creator deity. Or so I figured. But generally, yeah, I'd assume nagas can be fed off by a vampire for the purpose of feeding.

And a vampire can try to feed off animals to trick their hunger, but not to gain the actual benefits of drinking blood. If keri were to say, mortally injured and needs to drink, she can survive off of animal blood for a few days to prevent herself from going feral due to her injuries. But drinking animal blood won't help her heal her wounds any quicker nor would it actually help make her feel less hungry. 

I'd compare it to say you wanting to eat some real food, but all you're doing is drinking water. Sure it's not bad for you (Relatively speaking), but it's not going to solve the problem you have right now.

If keri does drink the Cosh Blood, it may help her get over some of the more psychological blocks she's having, like the possible pain of having broken bones. But it won't heal them. It'll just let her ignore them as if they weren't broken.