If this sentence means "Where will you go in the summer vacation?", it should be: なつやすみはどこにいきますか。 With the subject "you",, it should be: あなたはなつやすみにどこにいきますか。 If what the sentence means isn't so, please let me know. About the difference between は and が, please take a look at this page: http://www.guidetojapanese.org/learn/grammar/particlesintro I hope this helps you.