Do the following in your .vimrc!
Set your color scheme to solarized!
Launch vim with light background during the day and dark at night.
if strftime("%H") >= 5 && strftime("%H") <= 17 set background=light else set background=dark endif
Source ~/.vimrc any time you save a file, so the background color will change if it becomes day/night.
if has("autocmd") autocmd bufwritepost * source ~/.vimrc endif