In this context it's not 'a vacation' but 'days off' or 'vacation days'.
edit: Actually 'le vacanze' is used quite a lot for 'the vacation' as in this sentence. However vacanze can also be 'days off' or 'holidays' as in 'Quante vacanze hai?'. So I think that's what is behind the use of the plural.
I have learned from another source (Learn Italian For Travel....I love this ladie's videos and explanations) that the Italian "a" can be used to say the English "to", "at" or "in". The Italian "in" is used only for the English "in". I do not know how to attach a hyperlink but I find her on YouTube. Try copying this to your browser for her video on simple prepositions: https://www.youtube.com/watch?v=oUWgXUuhB_M