I am currently running 245/45/18 In front and 265/35/18 in rear. Which I think is causing slip light and vdc to kick in ? Should i upgrade the rear to 265/45?

 

26.68" diameter Front
25.30" diameter Rear

Given that the tire heights are backwards, yes. (Assuming it's a VDC car which it must be.)

I'd suggest just rotating them but it's still a bit outside of 3% ratio and if your Z is anything like others, the staggered wheel width is of no help there. I would suggest better sizes all around, 245/40 F and 275/40 R. The 265/45 is too tall and while it will work OK with that "rear tire being run up front", not optimal and may still trigger VDC error.

So many people think that going with a 35 aspect is "good". Sure, if on 19's; elsewise, stick with known stagger ratio with 40s.