No, you don't need to purchase Vista again for a hard drive. Just replace your drive then either:
1) Re-install XP, then upgrade it to Vista again or
2) Do a "clean install" with your Vista upgrade. Instructions here: http://www.dailytech.com/article.aspx?newsid=5932
Personally, I prefer the "clean install".
As long as you're just replacing the drive, you shouldn't have any problems.
I reinstalled twice. You do not have to buy vista again. Installed over xp pro then went fully to windows vista due to the fact I only had 60gb hard drive on my laptop. Next week I will reinstall again with a 250gb hard drive.
Just be prepared to have to call MS to get a new activation key.
I replaced my single C:\ drive with a RAID 0 C:\ and I had to call MS to get new activation ID. THe automated activation system didn't let me do it; I had to talk to a actual person.
You do NOT need to buy a new copy of Vista if your hard drive dies. I've got an OEM copy of Home Premium that I installed once and then reinstalled twice, once on a different hard drive. You'll have to call home to MS, but all they'll ask is if you have Vista on more than one computer.
Just tell them what you're doing and why, and tell them you only have one computer, and they'll give you a new activation key.