Нужен - male form
Нужна - female form
Нужно - neutral form
Нужны - plural form
Нужно is used with nominative version of nouns. In this case it plural form of овощ which is овощи.
Мне нужен отпуск (I need a holiday)
Нам нужна собака (We need a dog)
Тебе нужно время (You need time)
Им нужны книги (They need books)
Generally we use "which" when there is a limited number of choices otherwise use "what". Without a context we don't know.
E.g. we're in the kitchen. What vegetables do you need? Now, we're in the market faced with a selection of vegetables. Which vegetables do you need?
Yes that would be more correct, but in English many people use "what" to mean "which". It's colloquial but not truly correct