Any help is welcome , I was also wondering why there are such little choice out there for 18"
Few people use them. Also by going to 18" all around and keeping the correct wheel/tyre diameter you end up in some pretty oddball tyre sizes, so availability becomes (even more of) an issue. Wheel arch size limits means you can't go very wide and the number of 'narrow' 18" tyres is quite low.
Tyre sidewall also becomes very small, so it also becomes quite easy to ding/damage wheels on potholes and road imprefections.
is it really such a no in the way off mods ?
It's simple.. On a lightweight sportscar like the VX most modifications tend to focus on either make it lighter or at least keep it around the same level.
Bigger wheel+type combinations (unless you go for expensive fully forged wheels) tend to be heavier and they often put a negative impact on the ride and handling of the car (heavier, smaller/stiffer sidewall, etc.) unless a decent amount of time and money is also invested in a new suspension setup that can cope/use with the change.
VX'es are mostly 'driver'/track/road cars and not 'show'/bling/boulevard-cruiser cars. There are a few around, but they are rare and not many people go that way.
Of course anyone is free to do to/with their car as they see fit, but 'big wheels' is a modification that's not done often unless for specific reasons (eg. huge power needing very wide tyres) and usually with (lots of) additional bodywork and suspension work to make it fit and work.
It can be done. No question about that, but there's no 'commonly used' solution as there's usually little interest in it.
Bye, Arno.