If you merge the split files before importing them into Vegas you won't have that issue w/ missing frames. It's the same if you were importing a DVD you need to merge the VOBs into one big VOB first to avoid gaps being introduced. I'm pretty sure it's a sector boundary issue.
There're a bunch of free mergers on Videohelp.com - for example: http://www.videohelp.com/software/tsMuxeR
If you still encounter drift just apply a time stretch (CTRL/drag) but you should have issues with gaps at the splits after merging.