Spacing in alignedat

Since you need to choose the alignment point anyway, I propose a different approach, where you state the desired width of the object (default 3em):

\documentclass{article}
\usepackage{amsmath}

\renewcommand{\vec}[1]{\mathbf{#1}}

\newcommand{\splitcase}[2][3em]{%
  \makebox[#1][l]{$\displaystyle#2$} &
  \settowidth{\dimen0}{$\displaystyle#2$}%
  \addtolength{\dimen0}{-1em}%
  \hspace*{\dimen0}%
  \\
}

\begin{document}

\begin{equation*}
(G^{*} \vec{x}^{*})(\vec{w}^{*})=
\begin{cases}
\splitcase{
  -f_{0}^{*}(\vec{x}_{0}^{*})
  -(f_{1}^{*} \upsilon_{1}^{*})(\vec{x}_{1}^{*})
  -\dots
  -(f_{m}^{*} \upsilon_{m}^{*})(\vec{x}_{m}^{*})
}
  &\text{if $\vec{u}^{*} \geq \vec{0}$ and
         $\vec{x}_{0}^{*}+\dots+\vec{x}_{m}^{*}=\vec{x}^{*}$},
\\
-\infty &\text{otherwise}.
\end{cases}
\end{equation*}

\begin{equation*}
(G^{*} \vec{x}^{*})(\vec{w}^{*})=
\begin{cases}
\splitcase[5em]{
  -f_{0}^{*}(\vec{x}_{0}^{*})
  -(f_{1}^{*} \upsilon_{1}^{*})(\vec{x}_{1}^{*})
  -\dots
  -(f_{m}^{*} \upsilon_{m}^{*})(\vec{x}_{m}^{*})
}
  &\text{if $\vec{u}^{*} \geq \vec{0}$ and
         $\vec{x}_{0}^{*}+\dots+\vec{x}_{m}^{*}=\vec{x}^{*}$},
\\
-\infty &\text{otherwise}.
\end{cases}
\end{equation*}

\end{document}

I wouldn't change much about the setup of the first equation except (a) make sure the - symbol at the start of the math rows is typeset as a unary symbol, (b) change \begin{alignedat}{3} to \begin{alignedat}{2}, and (c) leave a bit more whitespace between rows 2 and 3. Whether to use arrow accents or bold-facing a vector is largely a matter of taste; if you use the arrow method, be sure to leave a bit of space before the superscript * symbols.

The spacing around the curly brace is unchanged if one uses \begin{cases} ... \end{cases} instead of \left\{ ... \right.

\documentclass{article}
\usepackage{amsmath} % for 'alignedat' env and '\text' macro
\usepackage{bm} % for '\bm' macro
\begin{document}

\begin{align*}
(G^{*} \vec{x}^{\mkern1.5mu*})(\vec{w}^{\mkern1.5mu*})
&= \left\{
\begin{alignedat}{2}
&{-}f_{0}^{*}(\vec{x}_{0}^{\mkern1.5mu*})
  &&-(f_{1}^{*} \upsilon_{1}^{*})(\vec{x}_{1}^{\mkern1.5mu*})
    -\dots-(f_{m}^{*} \upsilon_{m}^{*})(\vec{x}_{m}^{\mkern1.5mu*}) \\
  &&&\text{if $\vec{u}^{\mkern1.5mu*} \geq \vec{0}$ and 
              $\vec{x}_{0}^{\mkern1.5mu*}+\dots+\vec{x}_{m}^{\mkern1.5mu*}
                =\vec{x}^{\mkern1.5mu*}$}, \\[1ex]
&{-}\infty 
  &&\text{otherwise}.
\end{alignedat}
\right. \\ % end of first equation
(G^{*} \bm{x}^{*})(\bm{w}^{*})
&= \left\{
\begin{alignedat}{2}
&{-}f_{0}^{*}(\bm{x}_{0}^{*})
  &&-(f_{1}^{*} \upsilon_{1}^{*})(\bm{x}_{1}^{*})
    -\dots-(f_{m}^{*} \upsilon_{m}^{*})(\bm{x}_{m}^{*}) \\
  &&&\text{if $\bm{u}^{*} \geq \bm{0}$ and 
              $\bm{x}_{0}^{*}+\dots+\bm{x}_{m}^{*}
                =\bm{x}^{*}$}, \\[1ex]
&{-}\infty 
  &&\text{otherwise}.
\end{alignedat}
\right. % end of second equation
(G^{*} \bm{x}^{*})(\bm{w}^{*})
&= \begin{cases}
\begin{alignedat}{2}
&{-}f_{0}^{*}(\bm{x}_{0}^{*})
  &&-(f_{1}^{*} \upsilon_{1}^{*})(\bm{x}_{1}^{*})
    -\dots-(f_{m}^{*} \upsilon_{m}^{*})(\bm{x}_{m}^{*}) \\
  &&&\text{if $\bm{u}^{*} \geq \bm{0}$ and 
              $\bm{x}_{0}^{*}+\dots+\bm{x}_{m}^{*}
                =\bm{x}^{*}$}, \\[1ex]
&{-}\infty 
  &&\text{otherwise}.
\end{alignedat}
\end{cases} 
\end{align*}

\end{document}