The typical cast party to which a college student such as yourself would be invited would more likely feature jeans, than formal or semi-formal dress, but if you've been to one before, and you tell me it'll be formal or semi-formal, I guess I have to believe you. (Although note my comment on formal attire, below.)
Wear your white shirt. Pull on a pair of chinos. Buy an inexpensive navy blazer off eBay. There. The bulk of your work is done, and you haven't spent big money, either.
I'd add a tie, maybe even a bow tie. Almost any tie will be at least tolerable with a white shirt, chinos, and navy blazer.
If you own a pair of loafers, wear 'em. If not, well, shoes that'll work with chinos and a navy blazer aren't all that hard to find; you might already have a suitable pair.
The above is much better than the vest and grey pinstriped dress pants look you mentioned. It'll serve you adequately assuming most of the other male guests at the party are wearing sportcoats, blazers, or casual suits. It won't work if the party's truly formal dress, but I'm going on the assumption that what you call "formal," most of us here would identify as "semi-formal." (If you're not expected to show up in a tuxedo, don't go calling it formal. If you are expected to show up in a tuxedo, then your problem of what to wear is easily solved - wear a tuxedo.)
My experience with cast parties is decades old, and even then it wasn't all that extensive. But generally speaking, they weren't affairs where the most traditional aspects of proper dress were rigidly upheld. Struggling young actors weren't made to feel like buffoons for wearing balmorals instead of derbys, and that's putting it very mildly. Maybe things have changed, or maybe this is a cast party that's going to be covered by tv cameras and the press or something, and super-rich financial backers will be second in abundance only to middle-aged heiresses shopping for new boy toys. But assuming it's not that type of party (a pretty safe assumption), my guess is that you needn't get too stressed out over what to wear.