As far as I know will do nothing to help with Vegas.
Most of these kinds of cards hive the encoding task off to hardware, there's some very expensive ones that are very, very good for encoding mpeg-2 but they can run to $1000s
With Vegas a faster CPU is the simplest solution. With these encoder cards you're kind of stuck with them, even as CPUs run faster they still run at the same speed. The new Core 2 Duo CPUs seem to light a fire under Vegas.
I guess you mean the mpeg card that canopus has made. I am pretty sure taht it is not supported by Vegas. I remember it was made some years ago, when canopus did use Adobe Premiere together with their capturecards as an add-on to their solutions.
Definitely not supported or needed by Vegas. As farss said, a faster cpu is your best bet, but if the idea is to encode or render quickly so you can continue editing, and if you have another pc hanging around, you can always setup network rendering and offload the encoding job to another computer.