Thanks for the suggestion - what I ended up doing was this on load:
let vim_colors=system("cat ~/.bspwm_theme | grep VIM_COLORS | cut -c 12-")
execute 'colorscheme ' vim_colors

and then for the real time color changing I tried the above in a function which would get called on every write - turns out vim colorschemes do some very funky things when changed, so I think I'll just keep it on startup for now.

>That's awesome.
Thanks!