documentclass[12pt,a4paper]{article}

usepackage{ifmtarg}
usepackage{nicematrix}

makeatletter
    defbackify#1|#2@nil{%
        @ifmtarg{#2}{%
            #1%
        }{%
            #1 \ backify#2@nil%
        }%
    }

    newcommandvcoord[2]{%
        begin{pmatrix}
            backify#1|@nil %
        end{pmatrix}%
    }

begin{document}

$vcoord{3 | -4 | 0}$ but the formula is not closed..

end{document}