Is this question incorrect?

The reflexive past participle agrees with the subject noun ‘vacances’ - feminine and plural. ‘Se sont passées …’ is correct.

For the 2nd part, do you mean in answer to this simple question in passé composé specifically ? 

If effectively replying just ‘well’ or ‘not well’, passé composé is all that is needed.

It is certainly ‘possible’ to frame a response which includes imparfait. However, any answer that included imparfait would mean answering a somewhat different question, or going into a detailed discussion with background information etc 

As far as I have learnt and seen, es/ées will be only used when there is a previous mention of the subject. Like les pommes qui j'ai mangées étaient mûres. However, you didn't include subject in the sentence it should be "comment il se passait vos vacances?"

That depends, but as far as I have learnt and seen, in most of the cases french uses imparfait form, some passive forms and simple conversations use passé composé 

Kuldip - you are using an example of conjugation in passé composé of a verb (manger) with the auxiliary "avoir",  with the direct object preceding the verb. This is not relevant to the sentence Andrea posted for comment. (See 2nd link - from this site on special cases of avoir agreement)

There are a number of errors in your post that are relevant to understanding :

1. '... qui j'ai ..' as you used in your example sentence is wrong - it should be '.. que j'ai ....' and 'que' is the direct object (as the relative pronoun for repeating 'les pommes') - and therefore agreement is required with the direct object.

2. If you want to add to, or replace, 'vos vacances' (the sentence subject in the example Andrea posted) with a pronoun, the pronoun will be 'elles' - feminine plural just as for the noun it replaces.  'Il' doesn't make sense in this sentence whether as the masculine singular, or as the indefinite, impersonal pronoun.

3. Passé composé is actually the more commonly used past tense - imparfait is used regularly with étre and avoir in past tense, but in everyday speech, imparfait is less commonly used to discuss 'life events' than passé composé (See first link below) 

Laura Lawless does note on her site that 'imparfait' is used more in French than is 'past progressive' in English but that is not the same as saying that imparfait is the most common past tense used.

4. French students are largely taught to try to avoid passive structures, and while they are used, it is not as common as 'active' voice. (See 3rd link below) 

The final link is to a good discussion and demonstrations of the use of passé composé and imparfait that is helpful in developing a 'feel' for how they are used. I think it is very useful, but there are many other good online resources as well. It is a difficult concept for English background non-native speakers, as there is no complete and specific match in English.

The sentence Andrea posted, written I expect by a native French speaker, is correct in passé composé, and the agreement as shown is correct. 

https://www.lawlessfrench.com/grammar/passe-compose/

Special cases when the past participle agrees (in number & gender) when used with 'avoir' in the compound past in French (Le Passé Composé) 

https://francais.lingolia.com/en/grammar/verbs/passif

 https://www.frenchtoday.com/blog/french-verb-conjugation/passe-compose-versus-imparfait/