That's not true. You can swap front to rear on the 18's by removing the bolt on the front rotor hub. Very easy to do. It's only there from the factory so that people don't accidentally install the rears on the fronts and vice-versa due to the different tire sizes. Aside from the bolt (and the dimple on the inside face of the front wheels to accomodate the bolt), the hubs and wheels are the same.

 

You may want to have the VDC system checked also by a qualified knowledgeable shop. It may be malfunctioning intermittently and causing your issues, not necessarily the tires.

Technically, having the same size tires all the way around or different brand tires shouldn't matter. I run the same size tires on my coupe (245/40/19 on the OEM Rays) with no issues. I believe my front tires are different than my rear tires also because I had the front tires changed last year.