Depends what you want to have on that partition.
Many people have just the o/s and installed programs, and keep all their data on a separate partition. 30 - 40 gb is enough in that case.
If you reinstall Vista, you can either just reinstall it to the partition it is already on - in which case the 7 partition will remain.
You would need to add an entry to Vista bcd to get the option to boot either Vista or 7.
Or, after reinstalling Vista, you could use Disk Management to delete the 7 partition if you no longer want it.
Alternatively, while reinstalling Vista, select Drive Options (advanced) and delete and repartition as you like.