All of Rubicon's vehicles are scaled to 1/56th. Their upcoming Vietnam range will be the same - though they have said that their infantry for that period may be a little thicker (whilst, at least from the pictures shown off, still sharing the general look of their WWII models).
If you're looking for 1/48th - 1/50th then that's what most other manufacturers are doing to be honest. Rubicon's just bucking the trend as that's their preferred scale, and started out doing WWII models for wargaming; where 1/56th is more the standard.
I use 1/56th scale vehicles exclusively - having sold off all my 1/50th models years ago. They pare much better with more realistically proportioned figures like the WWII stuff out of Rubicon, Offensive Miniatures, Perry Miniatures, Wargames Factory, Eureka, Reiver Castings, etc.