I think Bia Hoi just means draft beer in Vietnamese, but it also refers to a cheap draft beer that is sold in cafes across Vietnam. I noticed them more in Hanoi. In the old quarter of Hanoi there are a couple of cafes that spill out onto a busy intersection.
The places are popular with both foreigners and the locals and are good place to meet up with other travellers. Small plastic seats are set up outside on the street. A glass of draft beer is just 2000 dong (US $0.12), so you can get a pretty good buzz going for just a dollar. People come around selling snack food and there is guy selling kebabs nearby for just 10,000 dong (US $0.62).
Hanoi closes down pretty early. Most restaurants and bars close at around midnight. The police regularly come around to check that the bars have closed for the night. Midnight is pretty early so most bars just pretend to close down. They close the shutters and turn the music down. They employ people who stand around outside looking out for police.
If they get a warning that the police are coming, they turn off all of the power and tell everyone to be quiet. Once the police move on the party starts again. It seems like a pretty ridiculous charade and I saw at least once the police come into a bar and collect money.