I would go for Randy personally. I like Violet better, but Randy comes in the awesome VILLAINS PACK. Also, there are so many Incredibles characters that you already have a good choice for that play set. However, if you don't like the other characters, don't mind having just a single character for a set, or don't want to get triples, I say get Violet.
I reckon that the "unlockable vault" will be in every play set... I would wait to see if this is true, and if it is, get the one you are more interested in unlocking first.
Also note, there will apparently be a "girls pack" soon, which may contain Violet(but I'm sorry if I've forgotten something we knew about this).