Yes, they have the soft life vests- My 2 year old JUST learned to swim unaided in one this summer, my 5 year old does very well in them. Im not sure about the slide, but when we stayed at the Hard Rock Hotel at Universal, those were the only life jackets they were allowed to wear going down the slide (I had bought my own and the lifeguard made my 5 year old change into one of theirs). I would imagine it would be the same rule.
So, depending on the age and ability of your child, yes, they can swim alone and unaided in them (you dont even have to get wet! LOL) and they can go down the slides in them at Universal.
Hope this helps!
BTW- we'll be there 10/11-10/25 maybe we'll see you!
NancyL