Stretching them out on the wider rims wouldn't cause flat spots, but some tires have habbits of getting flat spots from sitting overnight. I would try running them at about 40 psi and drive a few minutes on the highway to see if the vibration changes or lessens when the tires get up to temperature.

And if the rear rims are wider (staggered set) then I would still put the correct width tires on for proper wear patterns anyways... Did you by chance get any printouts from the road force balance?

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