What's the widest I can go? Was thinking of spacers and lighter wheels for my vRS but if I just get wider wheels I can do without the spacers and without the added weight. But what is the widest tyre I can go? Standard vRS is 205 40 17 and a 215 will fit but that's 7" wide.

I think beyond 215/40 R17 you can't get the right profile.  I run 215/40 on an 8" rim and it sits nicely, with a little bit of stretch

I was running S3 wheels with 215/40/R17 tyres and they were pretty much perfect. I swapped back to the 7.5" wide (I think) 17" Skoda vRS Spiders and there is a little bit of bulge that protects the rim nicely I think.

I think beyond 215/40 R17 you can't get the right profile.  I run 215/40 on an 8" rim and it sits nicely, with a little bit of stretch

These are what I'm looking at. What do they mean by offset?

http://www.demon-tweeks.co.uk/performance/team-dynamics-alloy-wheels/team-dynamics-pro-race-1-2-alloy-wheels-set-of-4

Offset is the distance from the true wheel centre to the mounting face, smaller number moves the face of the wheel out towards the edge of the arch, a bigger number movers the face in towards the suspension components.

I have 17x7.5 wheels on my car, with a sensible offset of 35 (ET35) at the front, and 28 (ET28) at the rear, I use 205/40/17 tyres and have no problems with rubbing or clearance.