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.