I'm not going to repeat myself to much here - I've already posted about my views on the MOTU firewire interfaces (I own the 828). Do a search and you should find it. It will either work or you'll never get it to work (with Windows in gerneral, not Vegas specifically).
Vegas will make use of all the ins & outs, but there is an option you have to change in the audio preferences box (I'm not on my PC with vegas on it at the moment, but it's something like changing from 'Microsoft Sound Mapper' to 'WMD drivers'....have a look in the preferences and you'll see it).
You can then record all of the individual inputs onto seperate tracks, and set up multiple busses for the outs. The digital i/o (s/pdif or ADAT) will also show up in vegas.
I *think* they are... most sound cards seem to support this these days. But the problem is this: to get your softsynths running at low latencys, (ie playable) you'll probably need to use the ASIO driver with them (I'm thinking of NI stuff), but Vegas doesn't support/use ASIO drivers - I don't think you can access the MOTU using ASIO and (for example) MME at the same time. PLEASE someone correct me if this is wrong (it's not something I've tried to do).
I'm fairly certain that you can't use two ASIO apps and one soundcard at the same time (but then this currently isn't an issue...). If you are happy with you softsynths using something like DirectX drivers, you shouldn't have any probs. Might need a quick PC for things to run smoothly.
That could be a problem, I currently use a Soundblaster Audigy and I often use 2 or more Ni soft synth using asio drivers and record them in vegas directly.