The vibration is because the spare tire isn't balanced
When possible, change the full set of tires if they are almost the same wear. Changing two by two may not save you money.
I recently changed 4 tires 205/65/15, 94H, max 51 psi, 65000 miles warranty. Such rating tires by Michelin will cost >$100 per tire. I bought Falken for $48 apiece from a Chinese stire warehouse in LA (the same tire in Discount Tire selling for $76 apiece).
