I don't use Vegas but with Premiere Pro you can import the first png of the sequence, then you tick a box which sees them as part of a sequence. Then the whole sequence goes into Premiere and can be viewed as a clip.
This means you don't need to use virtual dub to convert them (which takes tonnes of time). It reduces the amount of space your movie takes up and it is also MUCH easier to preview your movie in the editor.
A quick google about Vegas importing a png sequence and I found this....
"choose the first frame of your clip,and tick import as image sequence,choose open,and type the right framerate that u used in your movie,then it should work:)"
Try that Free and hopefully it will work.