Daughter's Jetta

My daughters 1999 1/2 Jetta has an engine vibration at around 3500 RPM’s which is kind of bothersome since that is about where it has to run at Interstate speed limits. Other than that it runs great.

Is this vibration present when the engine is revved to 3500 RPM while the transmission is in neutral and the car is stationary?

If so, this could point to an engine performance problem or a faulty engine/transmission mount.
If not, this could a problem with tire balance, suspension component, etc.

Is it a metallic “clang” vibration? Like a bent, out-of-tune cymbal? Probably a loose or rusty heat shield over the catalytic converter or exhaust manifold. Free to fix. Break it off and throw it away. (just don’t park in any dry, overgrown grass fields and you will be fine)