Daniel A. Mitchell writes:

My grinder had a slight vibration, no worse than an average bench
grinder. I dressed the wheels with no improvement. I traced this to
three problems. The out of center bolt circle described above. The shaft
flanges were out of balance (the back sides were not machined). I
removed these from the spindle and trued them up on my lathe, and then
balanced them. Big improvement. I also find that the steel-backed wheels
were out of balance.

I think this must vary from unit to unit. Mine was quite good out of the
box. Typical Chinese quality control. Maybe the store will let you fire
up the inventory a pick a good one?