Yes, it has already being decided. The marine infantry is using the Type 63 and the PT-76, they are both old, so what was decided was to buy the Sprut-SD and to upgrade the PT-76 changing the gun to a 57 mm (from a 76 mm gun) so that it acts as fire support. One key elements of the choice is that it always has to be a fully amphibious vehicle for amphibious assault purposes. I'm not sure about the price, I think 3 million or so.
The truth is, one way to go for Vietnam is to just get light tanks for the army (not just for the marines) such as the Sprut-SD and have the advantages of mobility, etc and to arm them with trophy active protection ($200k - 300k), so that instead of having all the heavy armor, they just rely on Trophy for immunity against rpg / atgm weapons, because in the end, whether they are 18 tons or 30 tons, if they get hit by a 125 mm shell, they are gone no matter what. I think that gives a good balance and is very good for the type of terrain that VN has. Sprut-SD can be dropped from a plane with parachute by the way.