1)Buy Viettel, the network is stable everywhere, data speed is great, price is cheap
2) Any local telephone shop, just buy the prepaid sim card (junk sim) with value about VND100k or VND200k that is already activated and have credit to use for both data and call
3) Less than $10 for the sim with basic data to use locally. But $20 is better for a month normal usage. If call oversea (back to Australia), it is more expensive since it charges you about 7k-10k VND (around 70 cents/minutes).