Local prepaid sim cards don't work well once you move to another country. Either get TMO stateside or get prepaid for each country upon arrival at airport.
HK: 7-11 store at arrival hall, or go upstairs to departure terminal, look for 1010 store
HCMC: once you clear formalities, look left side for kiosk
Bkk: multiple providers at arrival hall, btw doors 5-7.
Each can be bought for as little as US$10-15 for data/txt/voice....there are dedicated threads for more details on various provider options.