Assuming that VN interlines with PG (and it appears they do) then you can check bags through to USM. The process will require you to go through transit immi/security at the A/B/C/D concourse confluence in BKK which will then dump you into the domestic area (A/B).

Upon arrival in USM you will proceed to 'International Arrivals' where you can grab your bags and go through customs (which will likely be nothing more than you walking out of the airport).

This process is in effect for pax arriving into HDY, HKT, KBV, USM, CNX, CEI. The allowance is based upon there being immigration/customs at these airports.