Hi. I'm running Vistax64 and W7 7201x64 in a bual boot on the same disk and I think the boot order sorts itself out with W7 being the first boot choice but you have to install Vista first.
Obviously you are in a different situation with W7 on a seperate HDD but it sounds like a boot problem.
As far as I know there are no known issues with the x64 build, it facts it is considered better and more stable than the x86 builds.
I certainly find it so.
All the rocket scientists on this forum have been up all night waiting for build 7230 to leak but they will be back on later.
In the meantime I would suggest you look at your boot order.
Are you getting a choice at boot between Vista and W7?
Try downloading EasyBCD and use that to arrange your boot.
Best wishes, John
Oh and by the way warmest welcome to the forum