I posted this in the Sony Vegas forum but I thought I would post here as well. The problem may be the Win 7 drivers for the Intensity Pro Card.
I was running Vegas 9.0c on Win XP and my preview display through the Blackmagic Intensity Pro card on my HD monitor worked quite nicely. I could preview on the HD display in various modes (Best - Half ect.)
Now I've rebuilt my system using Win 7 64 bit and Vegas 10c 64 bit and the display doesn't work so well. If I have my preview set to Preview (Full) it displays a nice image of the timeline when paused but when I hit play the display goes to black. If I choose Preview (Half) for example the HD display remains black.
In short I had a very satisfactory experience using the Blackmagic card in Vegas 9 on XP and not very good experience under Win 7 and Vegas 10.