Here’s a summary of what I needed to do to access the Google Vision API from Python using Termux on Android.

1. Get API key on laptop - Generate service account key JSON file using Google Cloud Console on a desktop/laptop
2. Transfer using Tailscale - Use Tailscale’s file transfer feature to securely move the key file from laptop to phone
3. Install grpcio via Termux package - pip install grpcio won’t work on Android. Use Termux’s native package instead:

# Don't use pip for grpcio on Android
# pip install grpcio  # This fails

# Use Termux package instead
pkg install python-grpcio

Complete Instructions

See gvision-text repository for detailed setup and usage instructions.