I guess I can get a crossfader by starting with some vca:s, an lfo and an inverter. But I understand that is not enough.
What do I need more and how shall I think?

One missing thing: a dual mult. You need to send the same signals (input and modulation) to the VCAs to get a smooth L-R panning action. But there's an issue here in how the panning itself would sound...

Stereo panners are set up (usually) with what's called an "Equal Power" crossfade. What this means is that as each audio signal approaches the center of the stereo platform, it starts to drop so that when the signal is at dead center, each VCA would be outputting a 3 dB lower signal than the actual peak...that, in truth, would be from the extreme hard-panned direction to about 35-40% of the way to center. But you need to do this because if both VCAs were outputting an unattenuated signal, the center 10-20% of the stereo field would be abnormally loud. By lowering the VCA outputs at each VCA approaches the center, the summed level comes out to be pretty much the same as a single unattenuated VCA's. The result is a smooth motion across the stereo field without a big over-level "lump" in the middle. This is also how you'd want to do this if the VCAs were panning a control signal; again, you don't want this "zone" in the middle of the pan where the CV levels go waaaaay up, as that same, smooth equal power crossfade is also useful in keeping control signals from driving other modules into difficult to control areas.

Now, as to how to do that...well, that depends on the VCA in question, plus your own patching methods.

Thanks again for taking your time. I do not have enough knowledge for understanding exactly what you mean. But I think I got the big picture.
The modules I can use is Doepfer dual multi A-180-2, Doepfer dual attenautor A-183-1, Veils Quad VCA, an inverter, Mixer, Maths and some LFO:s.

There are crossfading modules available if you don't want to dedicate your VCAs and other modules to creating a functional crossfade.

Now I am in a process of learning how to patch and understand my modules. One goal is to learn how to combine different modules so they mimic other dedicated modules like a crossfader. Not so much that I need these functions. More for learning how they work.
One invaluable source is ”Patch & Tweak” that for example have learned me how to build generative patches with my 6U x 64 rack although that is ”Impossible” ;-)
For crossfading I found a good algorithm in Disting and I am thinking of to get the Doepfer A-134-2.