I find KM Player is significantly better than VLC (I have been using VLC for years and years). I run it on my x64 Win 7 machine no probs. It has a lot more functionality and controls than VLC, and can play MKV files much better. VLC stutters with certain MKV's. It also has a basic interface if you don't want all the options.
If you want a true x64 player, media player classic has an x64 version. The interface is pretty dated though...