You can install all your 3rd party apps on another disk, but unless it's very old software there will always be bits in the common files folder and in the registry on your system disk, so when you reinstall, the 3rd party stuff will need reinstalling too.
I don't think Vista will accept being installed on a portable drive, but if you somehow managed it, when you boot it on new hardware, it will want to be validated again I think.
I don't think Vista will accept being installed on a portable drive, but if you somehow managed it, when you boot it on new hardware, it will want to be validated again I think.