Recently bought a 98 accord 4cyl auto, found this line capped off with a screw. What does this line go to?

 

That is the vacuum hose from the engine mount control solenoid valve.

1. The engine mount control system comes on Accords with automatic transmissions. It is used to control excessive vibration at low rpms (e.g. idle). At low rpms, the PCM sends a ground signal (-) to the engine mount control solenoid valve, which opens the valve.
2. If the engine mount was replaced before with an aftermarket solid mount, then they may be why it is disconnected and capped. (Aftermarket has both solid rear mounts and vacuum assisted rear mounts for replacement.)

 

wonder if that’s why the accord has stronger than normal vibration at idle

 

Stopped in drive the car will vibrate quite a bit. Steering wheel will vibrate. If you brake and gas a bit to raise the idle speed and the vibration goes away, then it is most likely an MT mount or 'solid' AT mount.