"¿Adóndefuistedevacaciones?"

This should absolutely be accepted.

I agree because although in English we would ask “did you go on vacation?” and then follow it up with “where’d you go?” It’s unlikely that we would say “Where did you go on vacation”...the more natural translation would be either “where did you go for vacation?” Or “where did you vacation?”

In early learning you want word for word understanding but the goal is fluency.

If the aim is natural/flowing translation those (above) answers should be accepted.

But that would be "para vacaciones," not "de vacaciones."

Adónde (to where) is used when the question asks about movement toward something. ¿Adónde vas? Where are you going?
Dónde is used when the question does not ask about movement from one place to another. ¿Dónde estás? Where are you? ¿De dónde eres? Where are you from?
Spanishdict.com has many good examples.

but what is the replacement of go in this spanish question

I was also red flagged for using "holidays" - most certainly a common expression in my part of the UK. This translation has been submitted to DL for consideration.

I agree. While I would never say it that way, I hear that from British friends quite frequently, and it doesn’t cause any confusion at all. You should report it.

I concur, it was marked incorrect for me Nov 8, 2018.

It is normal where I was born and lived in England and Scotland (Birmingham, York, Edinburgh.) To clarify for the course developers, in the UK "holidays" is used for a single vacation, even though it's a plural word. Probably because a vacation is usually several days, and a "holy day" refers to only one.
Marked incorrect 10/12/18. Reported.

I think of this a being a place where English is doing something weird. We really ought to need a “to” here, but “where” and “go” together magically imply it, so we usually leave it out.

For the most part, I would suggest trying to give your answer as naturally as you can without changing the meaning, because the expected answer tends to be in a normal English format. Trying too hard to match it word for word won’t serve you well, because that isn’t how DL looks at the answer to begin with.

When you get one wrong, look closely at whether or not there really is a difference in meaning, and if there isn’t, then report it. It’s the only way to improve the accuracy of the grading.

When I use the correct English "to where", duo lingo always says it's wrong.

Prepositions don't always match up between languages. De vacaciones is the standard way to say "on vacation" in English. Notice that it's plural too.

http://www.spanishdict.com/translate/to%20be%20on%20vacation

I put holidays since the Spanish word used is vacaciones and it marked it wrong.

You are.

While “fuiste” can be the past of either “ir” or “ser”, when discussing where somebody is/was, you would use “estar”, so that would have to use “estuviste” instead.

With “ser” ruled out, this has to be “ir” and be about going somewhere, not being there.

You use “estar” when giving the location where something is or was, but this is talking about going somewhere. The verbs “ir” and “ser” have the same form in the preterite tense, and it’s “ir” that’s being used here.

If you used “estar”, then it would shift from “where did you go” to “where were you”, which could make sense but isn’t the question that was asked here.

Let me just add that using "adonde" rather than "dónde" is why this question can't be about where one was and can only be about where one went.

According to Castellano Actual, both are correct (just make sure to include the accent mark that's found on interrogative pronouns; not a donde and adonde, but a dónde and adónde) http://udep.edu.pe/castellanoactual/duda-resuelta-donde-adonde-a-donde-y-en-donde/ "Adónde es un adverbio interrogativo o exclamativo que significa ‘a qué lugar’, e igual que el caso anterior, también es correcto escribir a dónde: ¿Y adónde iremos de viaje?"

I think you could say it is natural Spanish but unnatural English. Native UK English speaker here.

Yes, but “de vacaciones” is just the way you say “on vacation” in Spanish. Note that “buenos días” is literally “good days” and “buenas noches” is literally “good nights”. It is the meaning that needs to be matched, and the languages’ standard conventions overrule literalism.

Yes, and I was wondering about the same thing. I guess if you are in Europe you would be used to holidays

http://www.spanishdict.com/conjugate/ir

• “fuiste” = second person singular indicative preterite of “ir”, so it means “(singular you) went”.
• English has this weird thing called do-assistance when forming questions (and negatives), so you could also say that “fuiste” means “did (singular you) go”.
• “fuiste” never means “outside”. That word is “fuera” or “afuera”.
• confusingly, “fuera” is also one of the subjunctive forms of “ir”, but the subjunctive isn’t needed here.
• the problem with using “vas” is that it is present tense, so it means “(singular you) go” or “do (singular you) go”, but the English used “did”, not “do”.

Did you flag as correct?

I don't want to come off as too critical, but it's not incomplete, it's wrong (for a few reasons). "fuiste" is a form of the verb ser, not estar, which is the verb used for being with respect to location (so the "were you" in "Where were you on vacation" would be "estabas" or "estuviste", not "fuiste"). Second, the question word is "adónde", which means "to where?". "To where were you on vacation?" makes no sense. The "where" in "Where were you on vacation" would just be "Dónde".

No. See other responses related to confusing fuiste and estuviste here as well as that adónde requires motion.

It actually matters quite a bit because people that complete one course often take the reverse course for a deeper challenge. I am currently “learning English” as a “native Spanish speaker”, and the reverse is true for many people on here.