Vaca (VA-kah, lit. cow) is a pool of cash contributions to pay for goods or services that are later enjoyed by the contributors and some shameless free-riders. Se armó la vaca (lit. the cow was assembled) is a common expression meaning that a vaca took place.
La vaca is a typical feature of poorly planned parties (of which you see a lot in Mexico City). Avoid looking cheap and contribute generously when you have a chance—the crowd will appreciate it. And if you are the host, avoid vacas at all.