Jankily making relative line numbers in Vintage mode
VintageLines is available via Sublime Package Control.
If you prefer to always have relative line numbers on set the setting "vintage_lines.force_mode": true.
If you prefer a key binding toggle relative lines on and off you can set one up by using key binding code like the following:
{"keys": ["ctrl+t"], "command": "toggle_setting", "args": {"setting": "vintage_lines.force_mode"}}
Note that you should also set vintage_lines.force_mode in your User Preferences to your preferred default for the toggle to be effective in both command mode and insert mode.

