Hmm... now that I think about it, I'm not sure if our alcohol policy would have allowed a third-party vendor. My chapter was relatively small, and there were few enough 21+ sisters who drank that we never even bothered to ask.
I turned 21 early in my senior year. Both our semiformal and our formal that year were in hotels, and we were allowed to go to the hotel's bar, buy a drink, and bring it back to the function. And afterwards, we went drinking
At my senior-year semiformal, there was a bat mitzvah going on next door. We had a really good DJ, but no alcohol. They had an open bar, but their band played mostly big-band stuff (obviously mom and dad picked the music
). The kids kept sneaking into our semiformal to dance to our music. I wanted to arrange a trade - they crash our semiformal and dance, we crash their bat mitzvah and drink