IIRC the VW strut nut is 22mm... a 13/16 would probably work for a 21mm but not 22mm
It has probably been 10 years since I've done VW struts though. Used to be a regular occurrance until I got smart and stopped buying cheap Boge struts and used Koni Yellows on all my cars. (and then old GTIs and Sciroccos stopped being cheap used cars and actually became collectible...)
I'm not sure what size the nut is on a Mk3 but a 13/16 is actually almost too big to work but it does.
I'm not as familiar with earlier VWs but I know some of the platforms like Mk2 and Sciroccos used a special strut tool that looks like a long, hollow hex that has 2 nubs to lock into a castle nut(?). I've seen the tool (I have one actually) but it isn't needed on anything '93 or above so I really don't know it's proper use.
With the availability of cheap coilovers these days, no one wants to pay the premium for a quality name like Koni or Bilstien anymore. The only thing the majority wants is "moar low".
The one thing they do pay a premium on these days are rare, period correct parts. If you have any of your old aftermarket or OEM stuff around sell it during tax return season and watch the kids go wild!