Hey Bob,
Dunno how you'd quantify the level of vibration. If kept at around 1/3 throttle, it is tolerable for several hours of use. If brought up past 1/2 throttle, it is unacceptably high on both the operator and the machine. With stock tires, the ground speed is on the low side. Please see the thread labeled "Honeeey? Do these tires make my butt look big?!?" on my recent experience field mowing with one.