Webooked ours from home and we didn't inquire as to vests only. We took one snorkel and one mask. We rented 2 sets of gear at $29 each. The set consisted of snorkel, mask, vest, and fins. They also gave you a brief explanation of how to use them and where the 2 sunken vessels are located(but it is easy to find without that information).
We had 3 people out at a time. 2 kids with vests and my husband without. I also saw others without and no one said anything to them...but it may depend on the day/lifeguards so don't quote me on that.
I didn't see anywhere that said you could rent just a vest. If you have space I would bring one just in case. It was neat. I saw alot of fish and a stingray. It was right off the main beach and better than Nassau right off the beach like we did last time. I'm glad we did it becasue we also had a snorkel at a reef in Nassau but it got cancelled because of a lightening storm.
Carnival Fantasy 6/05
RCCL Sovereign of the Seas 4/07
RCCL Navigator 12/07
RCCL Navigator 11/08
RCCL Liberty 7/4/09