Vertically aligned \underbracket's in an equation

Question

\[
\underbracket[0.5pt]{\left(
\begin{array}{c}
a \\
b
\end{array}
\right)}_{v_1} \qquad
\underbracket[0.5pt]{\left(
\begin{array}{c}
a \\
b \\
c
\end{array}
\right)}_{v_2}
\]

gives

I would like both brackets to lie on the same line, i.e., sthg like

Answer 1

use

\underbracket[0.5pt]{\left(
  \begin{array}{c}
  a \\
  b
 \end{array}
 \right)
 \vphantom{\left(\begin{array}{c}a \\b \\c\end{array}\right)}
}_{v_1} 

EDIT: This might work

\begin{equation*}
  \begin{matrix}
    \begin{pmatrix}
      a \\ b
    \end{pmatrix}
    &
    \begin{pmatrix}
      a \\ b \\c 
    \end{pmatrix}
    \\[-1em]
    \underbracket[0.5pt]\qquad_{v1} 
    &
   \underbracket[0.5pt]\qquad_{v2} 
  \end{matrix}
\end{equation*}

(anyone know how to get dvipng to not to cut off so tightly?)

Answer 2

Please always post complete documents showing all packages used. (I only got mathtools due to other answers)

\documentclass{article}
\usepackage{delarray,mathtools}

\begin{document}

\[
\underbracket[0.5pt]{
\begin{array}[b]({c})
a \\
b
\end{array}
}_{v_1} \qquad
\underbracket[0.5pt]{
\begin{array}[b]({c})
a \\
b \\
c
\end{array}
}_{v_2}
\]

\end{document}

Answer 3

Following the OP's comment regarding the lack of automation in the solutions proposed so far, I propose this:

1. define a command called \tallestmathstuff corresponding to the tallest mathematical "object" in your equation; some trial and error is necessary to make sure that it actually corresponds to the tallest thing.
2. define a command called \myunderbracket, which is build on \underbracket but inserts the \vphantom{\tallestmathstuff} before the first argument.

Then, using \myunderbracket instead of \underbracket prevents unnecessary clutter (i.e. \vphantom's) in the code.

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{equation}
\newcommand\tallestmathstuff{%
    % 
    % Put your tallest math object here
    \begin{bmatrix}
        1\\
        2\\
        3
    \end{bmatrix}
    %
}%
\def\myunderbracket#1_#2{%
    \underbracket{\vphantom{\tallestmathstuff} #1}_{#2}
}
%
\myunderbracket{
    \begin{bmatrix}
        1 & 0 & 0 \\
        0 & 1 & 0 \\
    \end{bmatrix}
}_{A}
\myunderbracket{\tallestmathstuff}_{x}
=
\myunderbracket{
    \begin{bmatrix}
        1 \\
        2
    \end{bmatrix}
}_{b}
\end{equation}

\end{document}

EDIT: I have updated my solution so that \myunderbracket can be used exactly like \underbracket.