Strange about the M46 I could've sworn I'd read it in the 1.0 manual (I just checked, it isn't), hmm maybe it was in the promotional hype before release?
I don't think I'm confusing it for another game, unless, were there any Nam mods for bf1942?
I'm sure I wasn't the only one who tried and failed, reversing jeeps into them and pressing every key while manning them (same with the Tango).
I'm happy to take your word for it, if you are into the game code, Myth busted
It definitely could have been something in the pre-release promotional stuff, which might have come from original plans. It's easy to see how much the game changed from its first concepts to what we actually got when you look at this picture of what the first version of the Sheridan looked like very early in development:
You'd never guess it was the same game. Heck, the barbed wire looks more like what we got in the finished product than the Sheridan itself does.
The only other thing I can think of is that maybe you're mixing it up with is either the US helicopters' ability to airlift vehicles, including the M110 self-propelled artillery, or perhaps the mortars that the players can set down and pick up.
There was Eve of Destruction for Battlefield 1942, but despite the popularity, it's kind of a mess, and programming something like that would have been beyond the developer who made it. There's at least one WW2 mod for Bf1942, Forgotten Hope Secret Weapons, that has towable artillery, but no one's ever done that in a BfVietnam mod. It can be done, but as FHSW proved, on the Bf1942 and BfVietnam engine, it's really awkward and has janky physics - not to mention, if I understand the FHSW system, it doesn't involve actually hooking up to and towing an artillery emplacement. Towed artillery spawns hooked up to a vehicle, but it's not artillery you can use, it's a fake. "Unhooking" from the artillery actually removes the fake model and spawns a stationary artillery emplacement, and "hooking up" destroys that emplacement and creates a new fake attached to the vehicle. The end result is that each artillery emplacement is actually a different one, with full health and full ammunition.
I think it's likely the developers originally intended the gate on the Tango to be moveable, because even though it lacks the code to rotate, it IS a separate model component. If they had no intention of it being moveable, they probably would have made it part of the main hull model. It's possible that they couldn't figure out how to solve the physics issues, or they didn't have time because development was so rushed. The game had two entire singleplayer modes that were cut because they ran out of time, after all. And the physics of one vehicle carrying another in BfVietnam can be pretty weird. I just got finished developing a landing craft that can carry tanks, but it had a glitch that made vehicles fall through the deck and sink, so the solution was to add a full-sized aircraft carrier hull, make it invisible, and take away its collision physics, and that magically made the landing craft work correctly. I haven't tested it yet, but I think a similar solution could work for the Tango.