Hello Matthew,
while the proy method worked for a single clip, Vegas always crashed after 5 or 6 imported videos (no matter which clips). It seemed like Vegas just couldn’t handle the file sizes.
I just installed a Vegas 17 trial and indeed, there were no problems concerning importing files or playback (even 30fps at best quality is no problem). I will definetly upgrade now (was about time anyway i guess).
So thank you very very much! Of all the things that could have fixed this, a software upgrade is the one thing I expected the least.