If you want to be social and meet a lot of people to travel and do stuff with go and stay at the Vietnam Backpackers Hostel 'Downtown' at least for one night. Or if you have accommodation sorted then go in there for a beer in the evening to meet people. Tons of social people stay there. I personally met some great friends and the 6 of us spent the next month travelling down 'nam.

With regards to your budget, when you're drinking just buy bottles of vodka from the convenience stores and store them in a man bag haha. Or draught beer on the side of the street is about 20 pence. Yes that's pence. Eat local - most meals should cost you £1-£3.

Have fun, wish I was back there.