ok fixed it. what a pain in the butt.
options -> file options -> load -> encoding = "text"
options -> languages -> extension manager -> c -> encoding = "Text, SBCS/DBCS mode"
My font de jure "monaco" whilst being fixed width is not I guess, fixed with "enough" for it to count, so changing to consolas or andale mono makes the vertical line appear. The reason it did not appear the first time is it bizarrely had 50 copies of my working file open, and it put a vertical rule on only 1 of those.. I closed all windows, and opened one.. it appeared.
I've decided to go without so I can use the font I like the most.

Seems kind of arbitrary that you can only get a vertical rule on some fixed width fonts and not others.