Should i feel guilty about this?

You are right to feel guilty, not just for being underage, but for helping yourself to something that isn't yours. However, if you do continue to take some of their vodka, it is good if you don't add water. It will lose it's potency if you do. And trust me, you won't have to add much water before they realize it's been tampered with. I know, my daughter's boyfriend tried that with one of my bottles not too long ago. ;-)