It's very easy to get a pre-paid SIM card here with a Thai number (you'll need to show your passport). Getting a post-pay contract is harder as you usually need a work permit for that. But pre-paid is plenty for everything you'll need.
I believe technically you're supposed to apply in HCMC if you're in Denang. They will want your address in Vietnam so best to check on the embassy website and apply in the correct embassy for where your hotel address will be.
Anonymous participant 406 you have to show proof you're in the country you're applying in. I showed my Vietnam visa stamp, boarding pass and hotel booking.