How do the は and が function here?

Question

受験生は夏休みが勝負なのよ?

So since there is no verb here, how do the は and が function here?

Answer 1

• As you can see in this chart, な in なのよ is the attributive form of だ ("the copula"). And だ/です safely works as the predicate of a sentence. (e.g., 今日は日曜日です, 私は学生です)
• よ is a feminine sentence-end particle.
• Here both は and が are used in one sentence, and this can be understood in the same way as you interpret 私は猫が好きだ or 彼は背が高い.
• 勝負 here is a noun which means the same thing as 勝負【しょうぶ】所【どころ】, "critical point".

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.

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.

References:

• Usage of nan desu?
• What exactly is "なの" (nano)?
• What is the meaning of ～んです? — there is a good answer regarding (な)のよ, (な)のだ, (な)のです, etc.
• 私は猫が好き and 猫は私が好き
• how could a sentence end with (noun + "よ"?)

Answer 2

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.
Examples:
母はまだとても元気なの。
主人はエンジニアなの。

So the translation could be:
夏休みが勝負なの。
Summer vacation is when the fight is decided.

The sentence above is identical to
夏休みが勝負だ。

が is the subject marker.
は is the topic marker.

The full sentence is:
受験生は夏休みが勝負なのよ。
For the applicants, summer vacation is when the fight is decided.