Well, I am generally using Rubicon 1/56 scale decals.
I used two sets of Rubicon DCUA007 Willys MB decals (for the Military Police labels) and theoretically one Rubicon DCUS002 M4 Tank decal sheet (I ended up needing two as I messed up one of the US Army decals and its associated serial number).
The US recognition stars in front of the side doors use the Rubicon 8mm decals, the ones on the rear half of the upper hull are the 5mm ones - all from the DCUS002 sheet.
The registration number is from Rubicon's Jeep kit (I used two sets of transfers, the two from one set, unmodified either side of the bow and one split into USMC and the number and placed one above the other to the left of the rear door). The number is incorrect for the vehicle (and probably too large).
Okay, this is a Soviet T-55.
Rubicon Soviet Tank numbers.
Vietnamese Stars from I-94.
Rubicon have a new range of 1/56 Vietnam models, and hopefully the decal sets will be available separately.