The following code produces aligned equations.
\documentclass{article}\usepackage{amsmath, mathtools}\begin{document}\begin{alignat*}{3}a_{11}x_1&+a_{12}x_2&+\ldots&+a_{1n}x_n&&=b_1\\a_{21}x_1&+a_{22}x_2&+\ldots&+a_{2n}x_n&&=b_2\\&\vdotswithin{+}&\vdotswithin{+}&&&\vdotswithin{=}\\a_{m1}x_1&+a_{m2}x_2&+\ldots&+a_{mn}x_n&&=b_m\end{alignat*}\end{document}This is how the result looks like.
The middle \vdotswithin{+} controls where the middle vertical dots should be. I would expect the vertical dots to be aligned below the second + in the equation since the command is \vdotswithin{+} but instead the vertical dots are aligned below \ldots. If I replace + with \ldots, the middle vertical dots get shifted to the left and they are not aligned with anything as far as I can see.
Why does \vdotswithin{+} align the vertical dots below \ldots even though the vertical dots are supposed to be aligned below +? I hope my question makes sense.
Any help is much appreciated!