Edit in Vim...

There is menu item to ‘Edit in TextMate…’. But I do not use TextMate. I wish to add the option to edit in Vim… Or actually, it should be ‘Edit in …’ based on user’s preference.

Actually, Edit in TextMate… isn’t part of Scrivener, but is a third-party service added by TextMate. The trouble with adding any external Edit In… features to Scrivener for the text files is that you could lose all sorts of data, or corrupt it - comments and footnotes, Scrivener links and so on - because the external editors would know nothing of these things.
All the best,