My guess is that someone or something has ticked the "Strictly conform to AVI2 specification" box under Options|Preferences|General.

Make sure that this box is unticked and then do the renders again - I expect you will find that the new files' full lengths will be shown in WMP, etc.

Explanation: The original spec for AVI files had a limitation of 1GB in size. Pretty soon it became clear that a revised spec had to be created that would allow files to exceed this rather low limit so the OpenDML (AVI2) spec was created which allowed AVI files to get round the 1GB limitation. Unfortunately some applications (e.g. WMP) seem to have problems with the interpretation of this spec and do not properly recognise some perfectly correct files as AVI2 (OpenDML) files, and consequently these applications will only see the first 1GB of the file (which for DV is only a bit less than 5 minutes). But if you have the "Strictly conform..." box unticked (which is the default) Vegas will produce AVI2 (OpenDML) files that even these problem applications will correctly recognise as AVI2 and all will be well.

Mark

BTW, there's a rather handy and free little application on the Canopus web site that'll switch various wrappers for AVI files that have the same data content, can save a lot of time re rendering just to fix a simple header problem.
Bob.