256 color support for vim background in tmux

From the look of your .bashrc and .profile, the shells inside tmux are overriding the 'default-terminal' setting in your tmux conf. Something like this:

• tmux creates new shell with TERM=screen-256color
• .bashrc/.profile run, set TERM=xterm-256color
• vim runs, tries to use incorrect TERM for tmux

you can check this by running

echo $TERM

in a fresh tmux shell.

Tmux is relatively picky about having a terminal set correctly. If you can, set the term value in gnome-terminal's configuration, not in your .bashrc. Failing that, surround those settings with a check for "screen" or "screen-256color" TERM, and don't reset them in that case.

Tmux REALLY wants the terminal set to screen or screen-256color

As explained here, disable Background Color Erase (BCE) by clearing the t_ut terminal option (run :set t_ut= in Vim and then press Control+L to refresh the terminal's display) so that color schemes work properly when Vim is used inside tmux and GNU screen.

I've found a better way on this post. You can make an alias of tmux to tmux -2 which will force tmux to assume that the shell is using 256 color terminal.