Student or Learner
May I ask when you are coming back from the vacation?
Is that a grammatical and educated form?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
The sentence also works with no determiner before 'vacation'.
I do mean "acceptable in the educated circle".
If you believe in speaking differently to people you consider "educated" from the way you speak to everyone else, then yes. Just about anyone, except maybe a very young child, would understand what you mean so it's appropriate for everyone.
We normally refer to that simply as 'standard English' If we are dealing with a word or expression that is not likely to be used by an average, moderately well-educated person in normal situations, we use other labels.
May I be permitted to enquire about the date of your return from your vacation? - Very formal, stilted
May I ask when you are coming back from your vacation? - Quite formal.
When are you (coming) back from your vacation? - Standard
When you back? - informal.