Thank you for the information!
I confirmed with the vendor and he said that he may have been "conservative" with the 2/3 ratio. He then backtracked and said that you need 20% free space for VM snapshots. 
We are not thin provisioning so is it safe to say that only 10% free storage will be necessary for snapshot overhead? Also, we will be introducing an extremely large 1.4~1.8 TB DB server--are there any particular overhead needs with such a large VM?
We are using Virtual Center 2.5 with ESX 3.5 right now, but will be upgrading soon. Right now some of our storage volumes are fairly close to being maxed out. I guess with vSphere there are some controls in place to keep you from using up too much of your storage?
> Also let me take a guess the vendor was a storage vendor who might be interested in selling more disks - 
Excellent guess! 