Breaking equation in align enviroment to put textwidth matrix in separate line

Question

I have to put a textwidth pmatrix into an align environment. When using split, it destroys the centered alignment of the other equation.

Pseudocode:

\begin{align}
 \vec{a} &= \vec{b} + \vec{c} \\
 \vec{c} &= \operatorname{Matrix}(Variable) \cdot \vec{d} \\
 \begin{split}
 \operatorname{Matrix}(Variable) &= \\
 \begin{pmatrix}
  textwidth matrix
 \end{pmatrix}
 \vec{e} &= 2 \cdot \vec{a}
\end{align}

Without the split the equations would leave the page. With split the equations are aligned left except the right aligned split part.

The result should look like:

      XX=XX
       X=X
      XX=XXXXX
      XX=
XXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXX
      XX=XXXX

Answer 1

\noalign can be used to put unaligned stuff inbetween:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
  \vec{a} &= \vec{b} + \vec{c} \\
  \vec{c} &= \operatorname{Matrix}(Variable) \cdot \vec{d} \\
  \operatorname{Matrix}(Variable) &= \\
  \noalign{$
    \begin{pmatrix}
      \rule{.9\textwidth}{1pt}
    \end{pmatrix}
  $}
  \vec{e} &= 2 \cdot \vec{a}
\end{align}
\end{document}