This is odd phrasing. You'd usually say, where are you going on vacation.
That is what I would say too
It would be plausible, albeit less common, if someone on vacation called up a friend, who did not know they were on vacation, and then told them that they were on vacation, prompting the friend to ask where.
In that scenario, I would say "Whereabouts are you on holiday?" ... so I think this should be an accepted variant.
I could be wrong but wouldn't "mihin" (where to) suggest the future tense while "missä te" implies you are already in the vacation?
I think holiday should be accepted
As a suggestion, British call a vacation, a holiday. I didn't try it in case it wasn't accepted. Vacation is American english
In line with the other comments, maybe "where are you vacationing" works as an alternative?
So, what would the Finnish for "Where are you going on holiday?" be?