So since there is no verb here, how do the は and が function here?
Therefore this sentence basically is a feminine version of 受験生は夏休みが勝負なのだ, which is an emphatic version of 受験生は夏休みが勝負だ.
→ As for test-takers, summer vacation is the critical point.
→ Summer vacation is the most important term for students preparing for entrance exams.
According to "A Dictionary of Basic Japanese Grammar", page 322:
の: a sentence-final particle used by a female speaker or a child to indicate an explanation or emotive emphasis.
So the translation could be:
Summer vacation is when the fight is decided.
The sentence above is identical to
が is the subject marker.
は is the topic marker.
The full sentence is:
For the applicants, summer vacation is when the fight is decided.