It's seems to be totally up to the system Chief Pilot as to who upgrades, if you're doing a good job in his eyes and they need the captains, you will eventually have a shot, there is no seniority here, and up till now there is no mention of upgrade in the pilot contract.
Where do you take your FAA medical in Vietnam
Well I think you actually refer to the class 1 medical (FAA is a U.S. thing), it's given in a medical facility near the company headquarters in Hanoi, the certifying authority here is the CAAV, or Civil Aviation Administration of Vietnam.