Add to you .vimrc file
Add following lines to your .screenrc file
attrcolor b ".I" termcapinfo xterm 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm' defbce "on"
Also check that your terminal supports or has 256 colors enabled. At least gnome-terminals seems to have this by default. Enjoy combination of vim and screen with 256 colors. :)