Preserve last editing position in VIM

Taken from http://amix.dk/vim/vimrc.html

" Return to last edit position when opening files (You want this!)
autocmd BufReadPost *
     \ if line("'\"") > 0 && line("'\"") <= line("$") |
     \   exe "normal! g`\"" |
     \ endif

The "out of the box" .vimrc enables this with the statement:

source $VIMRUNTIME/vimrc_example.vim

You may just need to restore this statement in your .vimrc. In any case, see vimrc_example.vim and also see the line() function in the Vim manual for a discussion of how it works.