Sorry, I misread your subject line to mean a centered vertical line.
Centering text vertically is something else altogether! If you are dealing
with a single page or section, you can choose "Center" as the "Vertical
alignment" (Layout tab of Page Setup).
Alternatively, if the page is at the beginning of a document or section or
follows a manual page break, you can use Space Before to visually center the
page. This is actually preferable to using the Center vertical alignment
because the optical center is usually a little higher than the mathematical
center. This option also allows you to have other text below the "centered"
title (Center alignment would center the entire body of text).