I assume you are saying that Vista is the current C drive and you want to make 7 the C drive. The cleanest, best way is to completely reformat the drive and reinstall both OS but on the new drives (partitons) you want them on. This is necessary since you want to change the OS on the c drive to 64 bit and because the OS and programs have references in their files to those drive locations that need to be changed.
You could create a new partition for data only and copy all data there, then only reformat the OS partitions - although I suggest backing up the data just in case.
Yes its hard to get around it when moving OSs on or off the C drive because of all the internal program refereneces..
Reloading the OEM OS on the same system is not a problem, worst case you have to contact MS to authenticate again and they give you a new code.
On my system, with Vista OEM software. I installed a new HD and - while loading the OS - still left the software on the old HD just in case I ran into problems - but I did not and did not even have to authenticate to put on the second drive.
If it was a 32 bit i would say install it from your vista partition in your rc partition.
but since it's a 64 bit you i guess you will have to do it from bios,and install it on the RC partition,you may loose the vista boot,so install Easy BCD on your windows 7 part.to re-create a boot for it.