MKV is a container extension, and not a file format by any definition.

As such, it "could" contain any number of file formats and codecs, which "may" or "may not" be compatible with Vegas' licensed decoders.

I would expect to see some basic mkv container support in the forseeable future, probably will be limited to a few codecs at first (h264), but without chapter, titles, or multiple audio support..