I get the same on occasions; sometimes if I open Vegas and just let it sit before I start doing anything because I have to leave my desk and when I come back, Vegas is frozen the way described above. On other occasions it just freezes in the middle of doing something innocuous. In all cases nothing can be done but force power down; keyboard, Ctrl, Alt, Delete, waiting for an hour or a day, nothing else works but shut the computer off...
I have had this happen when one of my USB disk drives spins down due to a power saving algorithm. When Vegas tries to go to the disk, the disk doesn't respond. I imagine this could happen with an internal HDD as well, but I would hope not.