Whilst getting no further than some massages with extras, I thought the Vietnamese were quite assertive & that your advice to negotiate before in order to avoid a nasty argument afterwards is wise.
As for the fees, well Vietnam is still significantly poorer than Thailand & 1000000 Dong is equivalent to 1588 baht. So I would hope short time is towards the lower end of your 500,000 ~1,000,000 price range.
As for your students, how do you make sure you're not getting a closet rent boy who will ask for $150 after the event ?