Your existing sentence, with the comma after which, is fine.
In this case you're using commas to indicate an aside, or a parenthetical phrase - something that is not essential to the understanding of the sentence. The sentence:
Our party will be held in the cafeteria which is located inside the Students' Union.
...would be fine and comprehensible by itself. The added phrase:
for those who are unaware
...is an aside, a parenthetical phrase which adds some context and depth, but is not essential for understanding.
Stylistically, it may be helpful to ALSO have a comma before the which, to separate the two clauses, but without the parenthetical:
Our party will be held in the cafeteria, which is located inside the Students' Union.
I'd imagine it's this usage which is tripping you up. However, the comma before the which in this case is doing a totally different job. It's separating the declaration of the party's site from the declaration of that site's location. Both parts would make reasonable sentences:
Our party will be held in the cafeteria.
The cafeteria is located inside the Student's union.
Combining them into one is absolutely fine, but it is helpful to use a comma to separate the two clauses.
You COULD have commas both before and after:
Our party will be held in the cafeteria, which, for those of you who are unaware, is located inside the Students' Union.
That would be correct, but feels awkward. You CANNOT have just a comma before:
Our party will be held in the cafeteria, which for those of you who are unaware, is located inside the Students' Union.
This does not work.
An alternative might be to use a different mark to indicate the aside. A dash or actual parenthesis could work there:
Our party will be held in the cafeteria, which - for those of you who are unaware - is located inside the Students' Union.
This avoids the awkward double comma, while still picking out the parenthetical phrase.
Best Answer
Although the popular style manual Kate Turabian's A Manual for Writers (a concise version of the Chicago Manual of Style) does not directly address your specific case, the proper usage can be deduced from similar examples. When word(s) within the middle of a sentence are omitted, three ellipsis dots should be used with a preceding space (e.g., a word . . . has been omitted). If an omission is made directly before the end of the sentence, the terminal punctuation mark remains; the ellipsis comes after the punctuation mark (e.g., Is there an omission? . . .). It also shows the following example: "We are fighting for truth; . . . for freedom . . . ; and . . . for survival". It appears that the punctuation of the sentence remains intact, with the ellipsis dots merely taking the place of the missing words. I assume that the example with semicolons could be extended to commas as well, as the semicolons are mid-sentence punctuation (and are occasionally used to separate a list or series of terms). Therefore, if eliding a list of colors, "Violet, . . . Indigo, Red" is more appropriate. I believe for the series of numerals, the proper mathematical notation would be "1, 2, 3, . . . , 19, 20", with a comma before and after the ellipsis. There is an entire chapter (ch. 14) on mathematical notation (including elided lists) in the full Chicago Manual of Style, 15th ed., but unfortunately I do not have a copy available for reference. If you need a reference for mathematics in type, then I suggest you look through that chapter. (The Chicago Manual of Style is available in the reference section of most libraries.)