gistfile1.vim

"open A_test.X if current file name is A.X
"open A.X if current file name is A_test.X
function OpenCorrespondingFile()
    let d = split(expand("%"), '_test')
    if len(d) == 1
        let name = expand("%:r") . "_test." . expand("%:e")
    else
        let name = d[0] . d[1]
    endif
    exec 'vsplit ' name
endfunction

map ,, :call OpenCorrespondingFile()<C-M>