I'd say it depends on the level of detail you want for your models, and wether you are scenario or point driven.
My go-to for Vietnam would be Spectre Operations V2 - Fast, brutal and with rules that covers Off Table Assets like Arty Strike and Gunships - The free Jungle Supplement also expands on the setting and gives you rules for Punji-sticks, jingle-wires and other nastiness! The Campaign fram could easily be modded and the scenarios likewise.