Out of curiosity, does this work for you?
If I have two instances of Vegas running, and copy & paste clips from timeline 1 to timeline 2, all works well... UNLESS the clips are nested files. If the nested file isn't already included in the project media for timeline 2 then the copy fails and hangs the program.
I don't remember this problem in the past---I'm using Vegas 9.0e 32bit on Windows XP.