I have the drain vitality shout listed but it doesn't give me the option to unlock the shout with a dragon soul...is this a bug?

I do use improved dragon shouts and freedom of speech and never noticed this problem before.

Are you using the Unofficial Dawnguard patch? It fixes several bugs with Drain Vitality .... or, you have a conflict somewhere.

how do I find the conflict?

Either run tes-edit , or start unchecking mods, until your able to train that shout. The tes edit would be the surest way of finding it, but unless your familiar with the tool, it would take some explaining.

My advice, would be to uncheck every mod your able to, without ctd'ing and see if that helps. If your able to train it, then start ticking mods back until you isolate the problem. If you can't use the shout with your bare minimum amount of mods, I'd start looking those mods up on nexus, and seeing if other folks had those issues.

Also, are you using bashed patch? If not, I'd actually try that first, it'd probably solve your issue.