You definitely should use F6 to install the AHCI/RAID driver before installing Vista. However, if you didn't and want to switch from IDE without a reinstall it is possible with a little work. See my June 22 post in this thread: about10253.html
DaddyRabbit notes a shorter approach that he used for XP in his June 23 post in the same thread, but I don't think this would install the Intel Matrix Storage Manager, which is useful if you want to go to RAID in the future.
Note my June 22 post about having to reactivate Vista after switching from IDE to AHCI. Also, the usual caveats about backing up, this worked for me but no guarantees, etc. apply.