More than likely the wheel is slightly out of true. Any time I'm taking old foam off a wheel to remount donuts, there's always some foam that has to be taken off chemically, or with a sanding drum on a dremel.
Short of a machined OD wheel, you're not going to find perfectly round wheels. As all the wheels out there are molded, you're going to find they're all out of true to a degree.
As for your vibration, I would start by making sure the arbor is seated fully on the motor shaft, and that the pinch bolt is properly tightened. Then I would remove the arbor nut and see if the vibration is still there.
Just about every arbor out there has a bit of play in the threads on the arbor nut, so the arbor nut doesn't always tighten up square, whether you have a wheel mounted or not. That's probably where any noticeable vibration would be starting.
Privateer racing lots of stuff.
Remember kids, racing isn't fair!