In the snorkeling lagoon, there shouldn't be any inconsistency. If you enter the lagoon you must have a snorkeling vest. Even if you bring your own snorkeling equipment, you still may not enter the lagoon without a vest, but the vests are available at the lagoon for use free of charge.
If you go elsewhere to snorkel, I could see there being inconsistencies. After all, in other places such as Serenity Bay, plenty of people go out into the water with no plans to snorkel and the lifeguards can't simply make everyone going into the water wear a vest. So while there might be vests available at places such as Serenity Bay, I don't see how a rule could be enforced unless they made everyone that entered the water anywhere have a vest.
I'm pretty sure the 1st time my wife and I went snorkeling off the beach in Serenity Bay, we didn't have any vests on.
However, because they can be use completely deflated and therefore won't interfere with your ability to snorkel or dive below the surface, I would strongly recommend their use AND that you only snorkel using a buddy system. It's very easy from Serenity Bay to find yourself 100 yards from shore in 6'-8' of water. That would be a bad place to start choking on water you've accidentally inhaled and not have any help near by.
I even went to the expense of buying a set of vests for my wife and I so that we could always be using them... and not have to have any where you don't know who might have been blowing on the valve or know how well they've been cleaned.