Open Question: How do I get mp4 files to play in Sony Vegas?

Okay, so have some audio/video files that are in mp4, and they play fine on Windows Media Player, but when I try to load them in my Sony Vegas Pro 9.0, it just WONT work. I converted one of the files to avi, but that totaly screws the quality of the video. D: Is there a codec or something that'll make mp4 work with my vegas? Please help.