it will always be at the whim of immigration. I just recently did this at Jomtien and they required the letter from the bank which takes a week. Immigration checked every detail of the letter/statement which had to be in a particular format.
I am well aware of Tet but thanks. I was just trying to find out how uncontrollable it was. It appears that Vietnam is sporadic and under control. I haven't gotten any suitable answers about Cambodia yet.