\vcenter - Used to center the argument on the 'math axis',which is at half the height of an 'x', or about the position of a minus sign.
{ \vcenter #1}
\vcenter command centers the argument on the 'math axis',which is at half the height of an 'x', or about the position of a minus sign.
\left(\Rule{1ex}{2em}{0pt}\right)
\left(\vcenter{\Rule{1ex}{2em}{0pt}}\right)
\left(\frac{a+b}{\dfrac{c}{d}}\right)
\left(\vcenter{\frac{a+b}{\dfrac{c}{d}}}\right)