I am certain they aren't allowing the vest, or at least they weren't when we went in June.
If it's only your paranoia...she's going to have to swim on her own someday. The lifeguards at SAB are top notch. When we were there, there was one lifeguard in the chair at the bottom and one on the pool edge itself.
My DD is 6 and she could touch the bottom when she came off the slide. I think it's like 3.5 feet or something like that.That doesn't really matter for us since she's a fish and swims whenever she can.
Have her try it without the vest. Be at the bottom when she comes out. *IF* she has a problem, I bet the lifeguards beat you to her - that's how on top of things they are.
Seriously, don't let a vest "make or break" your stay there. If you are so concerned with her swimming ability, enroll her in a swimming class or have someone test her in a pool with the same depths.
Truthfully, I am surprised your DD would tolerate it at age 6. Maybe mine is independent-minded, but there is no way she would wear one. Well, I would never let her wear one either. I fully believe in swimming unaided down any big slide. Which is why my kids had to pass swimming lessons before they were allowed on big slides, whether at home or SAB.