usepackage{xargs}
usepackage{xifthen}

newcommandxvecidx[3][2= , 3= ]{
    mathbf{#1}^{
        ifthenelse{isempty{#2}}{}{scriptscriptstyle (#2)}
    }_{
        ifthenelse{isempty{#3}}{}{scriptscriptstyle #3}
    }
}

newcommandxpd[2][1= ]{
    frac{partial #1}{partial #2}
}