Between last year, when she was two, and this year, I've tried just about every type of swim vest/ inner tube/ beginning toddler floatation device imaginable, including many that I deemed totally unsafe (neoprene vests that did not keep her head above water, or allowed her to fall and float face down, for example). Finally, this year (at age three) she had great success with this one (which IS coastguard approved):
In all honesty, she wasn't ready for it last year. But now it works really, really well, and she's able to swim all over the pool in it, and it always keeps her upright with head out of the water. There seems to be no problem with range of motion, and the fabric doesn't stick to the skin, so it's easy to pull on and off. (Comes in lots of really cute colors and designs, too!)
Hope you have a great vacation!