I have the same spacers on the rear of my car (in 10mm, but with aftermarket studs and wheels), and I wouldn't expect a problem with them since they're through-hole and pretty simple.
Whoever installed the 15mm spacers must have installed the +20mm studs as well, right? Any chance something went wrong there and you have a messed up wheel bearing? (e.g. from banging the old studs out or new studs in).
Assuming the wheels and tires are fine, the next best guesses are the wheel bearings and basically any of the suspension joints, of which there are many. Maybe go back to the suspension shop that aligned the car before and ask them to diagnose the vibration? Since they have experience with this stuff, they'll probably pull on a few things here or there and find the culprit pretty quickly.
Given how new and low-mileage the car is, one wouldn't expect any of those things to have just worn out on their own by now, but something could've become damaged or abused somehow (especially wheel bearings from stud installation). Sometimes wheel bearings are easy to tell, if you jack the car up and pull on the wheel with your hands enough to move the steering back and forth. You kinda have to know what you're feeling for, though (looseness other than the actual steering movement, basically), and not all bearing problems show up this way.
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