Certain buildings "push" supply. How far they push supply, and how high a level they push it to, depends on which building(s) you build, and the terrain involved. For example, supply will move more easily through plains with roads on them, than it will through deserted mountains. Most, but not all, supply buildings are military. Naval/air bases, barracks, and supply depots are the most common examples.
As for Vietnam, I'm not sure who you're playing as. Are you North Vietnam? Are you France helping out your southern colony? The answer changes depending on this.