Student's vocabulary knowledge is a building process which occurs ______ (with time/over time). Which one is correct here? Or the both is correct?
Firs of all, the sentence is ungrammatical. Secondly, it's awkwardly phrased from start to finish. Where did the article a that should be in front of student go? Knowledge of any kind is not a process. It's also not at all clear what you're trying to say with the phrase a building process. It sounds like a process that has to do more with construction engineering or architectural design rather than with leaning new words. You would use the verb to occur to describe things that you can't control. Building your vocabulary, however, is a process that's completely under your control. So, I don't think occurs is going to work there.
I were you, I'd just rewrite this sentence entirely:
Building one's vocabulary is a process that takes time.