If your creating/encoding ac3 in Vegas, check the custom settings... You may be killing your volume during the encode.
There's info in this forum I think if you want to search. Might also search the forums at videohelp -- if I remember correctly there were more than a few threads dealing with what the various settings mean in or to a ac3 file.
Otherwise 1 easy way is to increase the volume for the audio track or bus in Vegas before rendering the ac3. A lot of folks prefer normalize, which simplified pushes your peak levels to the max [though you can control it much better in Sound Forge] -- be aware that this *might* be a little high for your average viewer & how they have their TV set. Might also look at compression/expansion in Sound Forge.