I think you've mostly answered your own question:
a comma always goes before 'which'.
More rigorously, The Chicago Manual of Style (16th edition, section 6.22) has
Nonrestrictive relative clauses are usually introduced by which (or who/whom/whose) and are set off from the rest of the sentence by commas [emphasis mine].
So yes, the sentence is correct (as far as CMOS is an authority) with regards to the comma in question. I can understand why the use of a comma to set off the relative clause might look "off" in such close proximity to a comma separating list elements—it could imply that the clause only modifies the last element—but there is no justification in CMOS for replacing any of these commas with semicolons. You might replace two with em dashes:
Food like apples, oranges, and bananas—which are rich with vitamins—are delicious.
But that arrangement still suffers from the appearance of having the relative clause only applying to the last element.
Alternatively, if you really want to keep the sentence in its current order and attempt to alleviate the ambiguity, you could break with CMOS and generally accepted usage, but you'd probably want to modify the same two commas:
Food like apples, oranges, and bananas; which are rich with vitamins; are delicious.
To me, though, that looks excessively contrived (but don't underestimate how much of punctuation theory is subjective!). I might suggest that you rewrite the sentence entirely to avoid the aforementioned ambiguity as well as another problem: It is difficult to discern the relationship you are trying to illustrate between a food's vitamin-richness and its flavor. Are all vitamin-rich foods delicious? Are there foods as similarly delicious as apples, oranges, and bananas whose vitamin-richness is irrelevant? Perhaps more context would make that clear, but it wouldn't hurt to rewrite the sentence anyway.
As far as grammatical correctness, you need to either change food to foods or the second are to is.