It sounds like your Honda Fit automatic transmission is experiencing a problem that is causing engine vibration and stalling when in gear. Here are a few possible causes for this issue:

1. **Low transmission fluid:** Check the transmission fluid level according to the manufacturer's instructions. Low fluid levels can cause improper transmission operation, leading to vibration and stalling. If the fluid is low, you may have a leak that needs to be repaired.

2. **Dirty or clogged transmission filter:** A dirty or clogged transmission filter can restrict the flow of transmission fluid, leading to issues with gear engagement and causing the engine to vibrate and stall. Consider replacing the transmission filter and flushing the transmission fluid if necessary.

3. **Faulty torque converter:** The torque converter is a critical component in the automatic transmission system and helps to transfer power from the engine to the transmission. A malfunctioning torque converter can cause vibration and stalling issues. Inspect the torque converter for any signs of damage or wear and consider replacing it if necessary.

4. **Transmission solenoid problems:** The transmission solenoids are responsible for controlling the flow of transmission fluid within the system. If a solenoid is faulty or failing, it can cause erratic transmission behavior leading to vibration and stalling. Have a mechanic diagnose the solenoid system to determine if any replacements are needed.

It's essential to address transmission issues promptly to prevent further damage to the vehicle. If you are unsure about how to check or repair these components yourself, it's recommended to consult a qualified mechanic for a thorough diagnosis and appropriate repairs.