I'm not really sure.
Windows XP was known to almost never work when swapping motherboards (but CPU's were normally okay). Yet now with Winodws 7, you can swap nearly anything and it'll work. Some people still say that you can't do it safely, but I had Win7 installed on an old system and then one day put that HDD into a COMPLETELY new system.. new motherboard with new chipset, new processor, new ram, new GPUs.. everything.. and it worked just fine.
Unfortunately though, I don't know whether vista will perform like XP or more like Win7.
I had once managed to install a new motherboard on XP without the need for formatting or repairing my windows installation. My advise would be to back up all your data anyway, and then just try it out.
Chances are that it would probably work.
Just my thoughts :].
i had vista on my mediacenter.
changed the mb+cpu and kept the old install.
vista was a bit shocked at the first boot, but after updating from the web and restarting a few times it worked ok.
rule of thumb would be, that if it starts at the first boot on the new config, it will work afterwards.
ps. you will have to reactivate vista again, most likely. i did it by phone.
I would also suggest removing old motherboard drivers etc. - this is general practice when removing and changing your motherboard. This way.. it doesn't try to load old settings, drivers yadda yadda yadda.
After changing the MOBO you will want to use the installation CD to run a REPAIR INSTALL -- that will make the OS do a scan of your hardware and install the correct drivers for the new hardware and remove the old components that are no longer present - After running a Repair install and rebooting to windows you will want to go into the device manager and check that there are no warnings showing from components not working or any conflicts and may need to reinstall any components that were not installed properly ( Vista actually does a pretty good job of installing the correct drivers but it is best to check them afterwards just to be sure they are working properly or if anything was missed )
NOTE : swapping the MOBO and running a repair install will most likely reset the activation so you will need to reactivate the installation afterwards with Microsoft so be sure you have your key !!