What is it with mixing beer and vodka that makes you so drunk?

I'm not a big beer drinker, I seldom drink actually but when I do I go all out. Anyway, I can drink say a bottle of vodka and not be too bad but give me just one beer after it and I'm p*ssed. But if I had say a whiskey or a cocktail or whatever after the vodka I'd be fine. How come? Thanks.