After searching I found the CfgVehicle pages - and they list the Offroad (covered) / (comms) as VIV-compatible. So I loaded up the editor and played around with them - and was able to fit quite a lot of small crates and even the barrels into them. Also: Although the page lists them as "Cannot be transported" I'm able to load a stuffed Offroad onto a HEMTT Flatbed.
TLDR: The base mechanis I'm asking for are already in the game - but only to a very limited subset - and without actions for loading cargo (currently one can only load "vehicles" as in "cars" - but not as in "crates").
Adding those "loading" actions in a rather generic way seems simple - but it would be nice if this would be added to the game itself without having to script it.
In addition my base request still is: Please also enable it for the regular Offroader (as there's no logic behind why an enclosed vehicle can load cargo while it's not enclosed base model can't) and the small Truck. Maybe it would be possible to have both at the same time: infantry and cargo with shared spots.
Also also: Please add a way to somehow chose the spot a cargo is added to. Currently the auto-add feature aligns the cargo from front to back and in the middle. Playing around with multiple cargo at once there's clearly space for adding two Supply/Uniform boxes.
Another example is when one adds a Basic Weapons first and then a Supply Box the later added supply box turns 90 - but if you remove the first added the supply flips back and you can't re-add the basic.
It's so a nice feature - and as it's part of the game for so long it's sad to see it never got any polish over the years.

If adding all this generic functionality to the game is too much, could you at least extend the setVehicleCargo with an additional parameter like "mem spot" (or whatever its called) so cargo can be "managed" by scripts?

I think it would be nice if configuration of ViV transport functionality was also read from mission config. This will allow mission makers to add ViV transport functionality to vehicles that don't support it (whose models will likely never be updated), overwrite, or add extra configuration of ViV transport to vehicles that support it.
A possible configuration for Van might look like this:

class CfgVehicleInVehicleTransport
{
    class C_Van_01_transport_F
    {
        class Cargo
        {
            ...
        };
        class Carrier
        {
            ...
        };
    };
};

Well, unfortunately I'm not quite sure where to put this or how to load this. Can you give me some hint on how to add this?

You don't need to put this config sample anywhere since my proposal has not been implemented. If it is implemented, as I wrote, you will be able to add such configs to mission config.

What proposal you're talking about?

This one: https://feedback.bistudio.com/T180317#2591720

Well - according to https://community.bistudio.com/wiki/Arma_3:_Vehicle_in_Vehicle_Transport it seems possible to just add the VIV capability to new and existing vehicles. In fact: using the config viewer in the editor the vehicles that can load others do have a VehicleTransport section which contains a Carrier node. And although I wasn't able to confirm it my guess is that most loadable cargo shares some common inherited VehicleTransport/Cargo section.

Question is: How to use the missionConfig to add a Carier node to existing vehicles so that they can load cargo? According to the screenshot someone somehow managed to use some flat surface as kind of carrier to load multiple vehicles on to it. If some few lines are only what's required (as both the cargo area and the exit point can be defined with coordinates instead of adding memory spots to a model) my question could be solved by someone teaching how to do it.

Well - according to https://community.bistudio.com/wiki/Arma_3:_Vehicle_in_Vehicle_Transport it seems possible to just add the VIV capability to new and existing vehicles.

This is impossible without updating vehicle configs either by updating the game or enabling mods.

Question is: How to use the missionConfig to add a Carier node to existing vehicles so that they can load cargo?

I've already given an example of a possible configuration: https://feedback.bistudio.com/T180317#2591720
A more specific configuration description will be provided if my proposal is implemented.

According to the screenshot someone somehow managed to use some flat surface as kind of carrier to load multiple vehicles on to it.

If you're talking about BoxLoader, it actually uses such surfaces. But for these surfaces to appear in the game, players need to download and enable mod(s). And, as mentioned on the BoxLoader page, such implementation is worse compared to when carried objects are attached to the vehicle itself.

Again, as I proposed, it would be nice to be able to add ViV transport functionality via mission config. Also its config starting from v2.08 support relative positions along with memory points.

It sure isn't impossible - I built a simple config-mod with just this and got what I want:

class CfgPatches
{
    class muuh_caro
    {
        units[] = {};
        weapons[] = {};
        requiredVersion = 0.1;
        requiredAddons[] = {};
    };
};
class CfgVehicles
{
    class C_Van_01_transport_F;
    class C_Van_01_transport_cargo_F : C_Van_01_transport_F
    {
        side=3;
        faction="CIV_F";
        displayName="Truck cargo";
        class VehicleTransport
        {
            class Carrier
            {
                cargoBayDimensions[] = { {0,0,0}, {10,10,10} };
                disableHeightLimit = 1;
                maxLoadMass = 200000;
                cargoAlignment[] = { "front", "left" };
                cargoSpacing[] = { 0, 0.15, 0 };
                exits[] = { {5,0,0}, {5,10,0} };
                unloadingInterval = 2;
                loadingDistance = 10;
                loadingAngle = 60;
            };
        };
    };
};

So it sure does work as a (config-)mod - question is: Can it be set as mission config without rely on a mod for config override?

It sure isn't impossible

Will it work for players who haven't downloaded and enabled it? No? Then this is a problem, as it limits players from connecting to servers that use this mod.

question is: Can it be set as mission config without rely on a mod for config override?

No.

Well, I'm not quite so sure about your pessimism - although it seems, by look at https://community.bistudio.com/wiki/Description.ext a simple mission config is not able to override base config.
But that's exactly why this feature request is about: The Van_02 (to avoid confusion with the "small Truck" named Van_01 for whatever reason) comes with what I'm asking for:

1. a simple switch to change between additional cargo vs additional infantry
2. can load cargo as well as can be loaded as cargo
3. can be slingloaded

All I request is to extent this function already in the game to both the Truck and the Offroader - or, what would even be cooler but I guess way more difficult: allow us to override the VehicleTransport config in Description.ext.

It's not like I try to avoid to use attachTo - and the VIV stuff is quite limited: The vehicles have it enabled all use the front/center alignment, which blocks adding cargo side by side, and I still have to write some lines for loading cargo, and I'm aware that add that stuff to existing vehicles will be a challenge - but at least requesting this system to get properly implemented opens up this system to the entire community - instead of me just writing a few lines for my own joy without offering others to participate from it.
As a Linux user I profit from the opensource community every day - and already gave it something back - the same I feel for A3.