Don't dismiss the odd things, like the fact old cars can have lots of varnish in their tanks and in their fuel lines. All is OK and such doesn't cause problems as such. Until you put in some E10, maybe to save a few dollars because you've spend lots of $$ trying to fix some things on your car. Then the varnish could loosen into the fuel and gets to the injectors. Such varnish crud can cause idle and start issues and anything up to full on injector blockages. It all depends on how much varnish crud and particle crap is freed up into the fuel by using E10 after running 91 for many years.
But alcohol dissolves fuel varnish so you can use that fact to your advantage. Question is how much alcohol is OK in a VZ, it's E10 capable so at least 10% wouldn't cause tank, line or seal degradation. More on that later...
If it's still running rough, consider doing he following. Get some empty 20 liter drums from your local car dealer or mechanic, usually for gratis (just make sure they are clean inside). Then when your car is almost empty, get 20 liters of e85 and 20 liters of premium petrol in those clean 2x 20 liter metal drums. Go home and pour this E85 + premium into your tank. The strong alcohol dissolves the petrol varnish that forms over the years. Don't drive it with this mixture in the tank, since it;s not E85 compatible, just leave it sitting overnight, ideally giving the car a periodic bounce to shake things up. Then drain as much out as you can out of the tank as you can. It would be ideal to remove the fuel pump via the under seat access (which i think the VZ has). It just makes it easier doing so AND you get a good look into the tank that way (to remove crud if present).
Dispose of the dirty fuel appropriately (put it temporarily into those 2x 20 ltr containers you have). Obviously do this job in a well ventilated area (not a closed garage) and don't smoke and don't wear synthetics that can cause a spark... keep a fire extinguisher handy... and don't work alone... normal safety precautions... Put the fuel pump back in the tank (new screen filter?) and fill up with some quality premium fuel. Could also be wise to put in a bottle or two of cleaner into the tank to help clean the lines and injectors. Run the car and hopefully, if the issue was related to varnish crud, it will clear and start to purr like a kitten.
Only issue to note with the above is whether the tank, lines and seals are E85 compatible, metal tank and lines should be fine but seals? I also don't know what type of tank, lines and seals are in the VZ but the VZ is E10 compatible. But as you've effectively got 20 liters of each, that equates to 70% alcohol (so it's still a high %). If your concerned, reduce to 10 liters E85 with 20 liters of premium = 40% alcohol, or 5 liters E85 to 20ltrs premium = 20% alcohol. I'd guess 1/4 of a tank of 20% alcohol wouldn't be that bad for the components after some 8 hours (and possibly the car will run on this though not recommended).
If you've looked into the tank and there is no crud or varnish, then maybe check the tank venting system and charcoal canister and it's solenoids are working as they should. I've read the tank venting system can cause idling issues if not working correctly.
Only other thing i can think of is off fuel trim can be due to intermittent injector wining issues, injector problems or cat converter contamination causing odd readings thus throwing the baby out with the bath water. These things will result in high fuel consumption. You can safely clean your own old injectors at home or pay someone professional to do it as they also check flow rates, etc.
