Caption Assistant has a settings file that might become corrupt if you have to force quit Vegas Pro while Caption Assistant is active.
To fix it, you should close Vegas Pro and delete the following file:
%AppData%\VASST\Caption Assistant\1.0\Caption Assistant.settings
You can open a Windows Explorer window and enter this into the path bar and delete the file manually from the file list:
Or you can open a Windows Command Prompt and use this command to delete the file.
del "%AppData%\VASST\Caption Assistant\1.0\Caption Assistant.settings"
Either of those methods should work. Then start Vegas Pro again and Caption Assistant should start normally.