It will not support any of the "real-time" features of the pro-one card.
But with Vegas you don't need it, Vegas does it all in software. The faster your computer the more stuff Vegas can do "real-time." Vegas just needs an OHCI firewire card. So if Pinnacle has OHCI drivers for the Pro-One card, you can use it with Vegas to capture and output video. I believe Pinnacle does have OHCI drivers for the Pro-one. I know they do for the DV500 which I have. It works fine with Vegas.
I don't believe you can capture analog from Vegas using the Pro-One breakout box. If you ever need to, you can just capture it through Premiere and then import into Vegas.
Some DV cams (most of them?) will receive an analogue signal and digitize it in real time so you can capture it with Vegas through your IEEE 1394 connection.
I have not done this myself, but have tranfered via tape: Record the analogue signal to my DV cam first and then capture it like a normal recording. That gives me more control over how the clips are organized.
Tor