The first reason is to say good bye to the old year and welcome the new year with cheers and beers to a successful year. The second reason is to meet up with friends and colleagues. The third reason, which the government is actually trying to stop, is the exchange of presents from the staff to the management, which is a common form of corruption.
Almost every company celebrates at a restaurant, Bia Hoi or at hot pot places, karaoke or at Dog restaurants. You might think that it's so weird to celebrate an end of year party at a Dog restaurant, but in Viet Nam, eating Dog meat at the end of the year is a way of ridding bad luck away from you and welcoming the best to come to you.
At this time of the year, it's not easy to book a big party in an expensive restaurant because they are already booked by all of the big companies for months in advance - even hot pot restaurants. Karaoke is popular after meals to relax for Asian people who love singing and can sing amazingly well. But along with the normal karaoke there is another second service called "karaoke om", which means you sing with a prostitute sitting right next to you, who provides extra 'services' in exchange for a big tip :)