OK, after experimenting, I have managed to get the FMC working in other aircraft now.
First of course you need to make a window for it in the panel.cfg file, something like this:
[Window06]//MCDU
Size_mm=377,600
visible=1
position=8
ident=GPS_PANEL
gauge00=vasfmcgau!mcdu,0,0,377,600
That gets the fmc into the cockpit (adding the title window too of course at the top!).
To get the gauge operative, I added this line to the end of [Window00], using next available gauge number for XX:
gaugeXX=vasfmcgau!nd,0,0,1,1, builtin id10002 a_style
I changed the dimensions and placement as this entry produces a duplicate Navigational Display gauge. 0,0,1,1 makes it invisible. Without this gauge in the panel.cfg file, the fmc does not switch on.
By the way, you don't need to add the four vasFMC folders _acs, _lgip, _rmp & _sys to the panel folder...
All you need then is an icon shortcut in the cockpit to open and close the FMC - easy enough with an xml script. (The toggle supplied with vasFMC didn't want to work, so I just created a new one).
I am not sure yet about the operation of this FMC and whether simply installing it into an existing panel.cfg is going to work (this is my first day using it), but maybe the above will help someone at least to get the gauge operating in the cockpit.
Martin