Can I suggest:
1) Abandon the FF-mpeg converter software - If you want to brighten the footage you can do so in the editor in the same way that this converter does it
2) Once you follow step 1, You'll be working with a high quality "original" file from Fraps which is compatible with Vegas rather than a compressed "fixed" version that is incompatible.
Hope this is helpful to you!
( if the video footage is too dark when you record it, Try increasing the brightness settings of the game? )