1) Is it possible to dual boot 32-bit Vista + 64-Bit windows 7?
2) Is it possible to dual boot 32-bit Vista + 64-bit Ubuntu Linux?
3) Is it possible to triple boot 32-bit Vista + 64-Bit windows 7 + Linux 64-bit?
It is definitely possible to do 2). I have dual booted Windows Vista/7 with Ubuntu without problems. The key is to install Windows first and Linux second.
1) and 3) should also be possible. I have a desktop rig that triple boots Arch Linux, Ubuntu, and Windows 7. To triple boot, you have to be a lot more careful about how you partition the hard drive. However, I've never done a triple boot where two of the OSes are Windows.