Budget FMIC?

... im concerned that the button in the dash wont fit in with the rest of the vRS, if it does, will it light up the same and have the dimming functions at night?

There is probably at least one blanking plate in your row of switches. I got a spare ASR pushbutton (push-and-hold action) part number 6Y0927133 47H for my klaxon. It fits quickly and neatly, and would light up and dim if I'd connected the appropriate terminals to the adjacent switch. It confuses everybody because I've now got two ASR buttons, of course. The heated rear window switch is a push-on, push-off type. You can't buy matching switches with no lettering on them.

For an intercooler spray, I would guess that you would want a switch you can reach from the steering wheel rather than having to fumble for one of several switches on the centre panel - because if you need the intercooler spray you are presumably busy with the driving.