How do you remove old Windows Vista Backups?

Well that was dumb. Pretty strait forward that.

On your backup drive look for a folder with the name of your computer (Start->Computer->Right click select properties)

Inside this directory here will be directories with a name format similar to:

Backup Set YYYY-MM-DD <6 digits>

Additionally on the drive, there may be a directory called WindowsImageBackup\<your-computer-name> and this will contain directories with the same name format as above.

Based on the date, delete the old backups.