Don't delete those files until you have done one of the following.

1) finsihed your project and rendered it to the final format.
or
2) save your project using the menu option of "File/Save As" and select the option to "copy and trim media with project". That will basically create an AVI file for the events in your VEG (and reduce it down to the 1.5 hours or so contained in it).

Option 2 may not easily work for you if the reason you are trying to do this is because you are very low on disk space... as of course by doing this you will initially need an additional space for the extra 1.5 hours of DV footage onto your drive. Once you have done this... you can however delete the original AVI files that you captured.