Hi Ben,
So far I've relied on the vim functionality that comes with lilypond (found in lilypond's 'vim' directory). I am simply more comfortable and feel more productive with vim's modal editing, navigation, visual block mode, buffers and windows, etc. It's also already configured to my preferences. I'm glad Frescobaldi supports split editors, but it kind of makes my head spin: the tabs don't line up with the editor panes; I have to think about if clicking on a tab or link will change focus to a different pane or cause a pane to switch to a different file, and in either case, the feedback isn't great. I'm currently working on a band project which I've organized into 9 different files, so multiple windows/buffers is an important feature. Setting lilypond as the make program enables the use of vim's quickfix list, which is very handy and lilypond includes multiple macros to support this. Reading ftplugin/lilypond.vim may give some ideas on further customization.
Using vim for lilypond comes with its own issues though. The biggest one I've encountered so far is that syntax highlighting can cause vim to lag, although this seems to be a general weakness of vim. It's somewhat better for me using gvim vs a terminal emulator. Automatic indentation is imperfect (lilypond 2.18.2 - I don't know if this has been touched in the development version). I don't own a midi keyboard, but if you rely on midi input I don't think that's going to work in vim. Automatic engraving is probably also not an option. Frescobaldi's definition linking is very useful, but I'm guessing this might be feasible in vim with ctags, I haven't tried yet.
The truth is, I'm very new to lilypond, so I'm still figuring out how everything works, let alone workflow - although I'm consistently getting better thanks to practice and a knowledgeable community. Your videos on lilypond and Frescobaldi have been very helpful, I wish I could return the favor!