Do you already have a DV500 card and are thinking of moving to Vegas?
The DV500 WILL work with Vegas. However, it will only work as generic OHCI firewire card.
So if you have a DV500 card already and you have the OHCI drivers for the DV500 you can use DV500 to capture video and preview out.
If you don't have already have a DV500 and you want to work in Vegas, do not buy it. The "real-time" editing features only work with Premiere and only earlier versions of Premiere, it will not work with PremierePro. On a reasonably modern computer Vegas has more "real-time" editing than Premiere + a DV500 card anyway. So you end up paying a lot of money for software you won't use (Premiere, etc) and hardware that gives you the same functionality you can find in standard, much cheaper cards.
The breakout box with analog capture will only work with Premiere, however, if you already have the card, you may be able to work out a analog to Premiere to Vegas workflow.
If you want to set up a new system, just get Vegas and good firewire card. I think people like SIIG and ADS, but check around. If you need analog capture, look into the Canopus or ADS firewire/analog devices.
If you have the DV500 and you have the OHCI drivers, just get Vegas. If you don't have the OHCI drivers, upgrade to them. You will have to reinstall the card, I think. Bottom line: if you don't have the DV500, Vegas users don't need to buy it. If you have the DV500 and want to switch from Premiere to Vegas, there's no need to throw the DV500 away.