Dear John, Nicola,

I created a file context.vim under the folder ~/.vim/after/ftplugin/ which contains

map <C-h>><Esc> :w<CR>:!ConTeXt<CR>
let g:context_mtxrun = 'PATH=$HOME/ConTeXtLM/tex/texmf-osx-64/bin:$PATH mtxrun'
let g:tex_flavor = "context"

The result is
1. using “:ConTeXt”   issued an error message:  “E492: not an editor command: ConTeXt
2. using “make” issued an error message: "No targets specified and no makefile found. …" 
Pressing C-h, compiling started and succesfully ended. However, it use /usr/local/texlive/… not LMTX tex folder.

Without context.vim, “:ConTeXt" and “:make" worked fine.

I’ll do more test. 

Thank you.
Best regards,

Dalyoung