I agree with the "Z" man above.
By rotational I'm assuming you mean asymmetric?

Regardless, if you're feeling a vibration anywhere from the rear end & the only thing that has changed has been the tire, well, I think it's safe to assume that it's the "new" tire they sold you.

Ask them to score you another Conti just like the one on the other side, have them balance it & take back the Hancook they sold you!

The "W" & "Y" ratings are speed ratings for the tire - the theoretical limit of speed that the tire will do without falling apart; "W" = 168mph; "Y" = 186mph. Since I wouldn't think you'd be doing over 168mph for any length of time I wouldn't worry !

Tire Tech Information - How to Read Speed Rating, Load Index & Service Descriptions