I'm sort of copying this from the Audio forum. Behringer have a 8 channel control surface just come onto the market for AUD 490.
This is WAY cheaper than anything else and would seem an ideal way to automate not just audio envelopes but also video.
Any fellow Aussies who are interested you can get one from here:
http://www.venuemusic.com.au/Products.asp?ProdID=4866
I don't have one as yet, (space is the problem!) so I cannot comment on how easy it'd be to set it up to work with Vegas but once someone has it 'cracked' the config files could be shared.
Bob.