How to put the dots in the centre?

One way is use matrix as defined in amsmath package:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
    \[
\begin{matrix}
A   +   B   \\
B   +   C   \\
  \vdots    \\[3pt]
Y   +   Z
\end{matrix}
    \]
\end{document}

or with array (based on the last solution stolen from deleted @campa answer):

\documentclass{article}

\begin{document}
    \[
\begin{array}[t]{r@{\:}c@{\:}l}
A & +   & B     \\
B & +   & C     \\
& \vdots  &     \\[3pt]
Y & +   & Z   \\
\end{array}    
    \]
\end{document}

Result is in this particular case similar as before, however, now terms on left and right from + can have different lengths but math expressions are still centered at + sign.