Besides the obvious risks, is there any harm caused to an unborn child by riding a motorcycle (vibrations) while pregnant (first trimester)?

Unlikely to matter. Once your placenta has a firm attachment in the uterine wall, the developing embryo/fetus is well protected against repetitive vibrations. I have had mom's teach their jazercise class for the balance of pregnancy & never miss a beat. Occasional mom's with an unstable pregnancy, odd placental attachment or other issue may be advised to maintain a quiet lifestyle, but that is an exception.