If you are going to a family meeting, there will be kids, so you'll probably find yourself having to let your console to at least one of them to try X game you have and he/she doesn't or play with or against some of them (if they also brought their handheld).

With that in mind, the question is: which console are you more worried that may get broken? And if you like the idea to play with some of them, which console is more likely for them to have?

Depends on what games you have finished. If you have completed games like Persona 4 and final fantasy on vita, I would say that there is no doubt that 3ds is the better option. Animal crossing and mario kart can give you many hours of fun even after playing them to death.

Your Animal Crossing town will look like shit if you leave your 3DS behind. Just saying.

Take the one whose battery last longer . I don't know about the Vita but my 3DS battery doesn't last more than 5 hours.

If i could only take one i would take the Vita. Its has multiple functions that work better (like video and music you can take in it aswell to listen and watch) or you can just surf the web and watch youtube if you have a wireless internet connection (or download stuff). The games are down to whatever you want to play. Though it does sound like you have a wider array of games for the Vita. The 3DS would be an understandable choice if you were hellbent on beating Tales of the abyss (im playing it now and im on 50 hours and still got ways to go).

Personally i have alot more games on my Vita than my 3DS. Though if say, it was pokemon or ace attourney launch week i would stick to my 3DS. Otherwise, Vita is a much better portable to have. If you have Persona 4 golden to complete still, then theres no doubt you will be kept busy.

Vita + Persona 4, you won't ever need anything else

Honest answer. I would say neither because vacation should always be used for things you can't do at home, like weird food and museums. Also, it is the most fun time ever to hit on ladies. But if you have to bring one, or you are a child, or already not single, Vita.

I never said that. I elaborate in another post, so check that one out.

Not bashing the Vita, it's a great system. But I would choose the 3DS if given a choice. 

Wow...  You're still going to feel that way rather you have one or both, then.  The only difference is, having one, you wont have the option to play the other while still missing it.  I'm not even going to contribute after an answer like that.  Bring whatever you will.