I am sure that there is no real chance to get an upgrade. Go ahead and ask the agency offering you the contract to put it in your contract, I think they wont do this.
Some pilots got a change from 767 to 777 but this was more or less something to keep the face of some highranking guys in the airline.
Remember, all the flying jobs for VN are contract jobs nothing more. Also keep in mind that this is Vietnam and the people are very proud and everything is organized acording to the liking of the party.
I would say there is no chance for an expat to get an upgrade.
Regarding the standards of flying within VN, I would agree with what was mentioned above, it's doubtfull.
I am glad that I could leave the place.