No.do the 16" wheels/tires offer a smoother ride?
Q: "do 15" wheels offer a smoother ride?"
A: No.
With our Vibes it is simply wrong to use the term "smooth". The Vibe, Matrix, Corolla suspension is harsh.
Higher sidewalls will reduce the harshness only marginally. The ride remains harsh.
I have new tires on my 16" wheels. They are quiet. I have 15" wheels with winter tires. They are a bit noisy but the ride remains harsh. They offer only a wee little bit of relief.
I was researching for ways to get a more compliant ride and found out that the rear struts have only about 5" of travel. This compares to 9" of travel on our Ford Contour which is not a prime example of smoothness either, even though it is much more compliant than the Vibe's suspension.
To me, the harshness of the ride is the only real downside of the Vibe.
All the best!