If this is TRUELY on a separate partition, then its very simple. And SHOULD not interfere with Vista.
1. Within System Properties\Advanced\Startup and Recovery - select Vista or Older operating system as the default
2. While in Vista, within DISK Management. You will see the partition that Win7 is on. Once you have verified that this is it, then right click on the partition graphic below and select delete partition. (But you must be sure! I have notice that my Win 7 sees its partition as C: but does not recongize my XP c: but does see it in Disk Management.
3. Restart
Is there anything wrong with Vista? What are the 'lot of problems' that you have. Please explain those prior to following the instructions above.