Even if you could edit the files with vi it probably wouldn't solve your problem because the file system is ephemeral. Meaning... If you edit a file via heroku run bash you aren't actually changing the file for other dynos. To change a file for all dynos you need to either change what you push in a Git repo or change the buildpack. More details:
https://devcenter.heroku.com/articles/oneoff-admin-ps#formation-dynos-vs-oneoff-dynos
