I noticed that, within screen, I was not getting the full syntax highlighting in vim that I was expecting. Something had to be up, and it wasn’t long before I found this tip on getting full colour support for vim in screen.

After so poking about, I only used a subset of those instructions. I added this to my .screenrc:

attrcolor b ".I"
termcapinfo xterm 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm'
defbce "on"

My .vimrc got updated with this:

set t_Co=256

It did a great job, and now everything looks as it should.