short way:

$ TERM=vtn102 vim

to define the commands:
:MonoTerm
and
:ColourTerm
---
function! s:ColourTerm()
	if has("terminfo")
		let &t_Co=8
		let &t_Sf="\e[3%p1%dm"
		let &t_Sb="\e[4%p1%dm"
	else
		let &t_Co=8
		let &t_Sf="\e[3%dm"
		let &t_Sb="\e[4%dm"
	endif
endfunction

function! s:MonoTerm()
	if has("terminfo")
		let &t_Co=0
		let &t_Sf="\e[3%p1%dm"
		let &t_Sb="\e[4%p1%dm"
	else
		let &t_Co=0
		let &t_Sf="\e[3%dm"
		let &t_Sb="\e[4%dm"
	endif
endfunction
com! ColourTerm call s:ColourTerm()
com! MonoTerm call s:MonoTerm()
---