Vectors and ClassicThesis (with eulermath)

The eulermath option loads the eulervm package, which distinguishes the standard \mathbf command from its own command \mathbold which uses Euler Bold.

So the answer is

\renewcommand{\vec}{\mathbold}

If you want a redefinition that's independent of the eulermath option, you can say

\ifthenelse{\boolean{@eulermath}}
  {\renewcommand{\vec}{\mathbold}}
  {\renewcommand{\vec}{\mathbf}}

_{Sorry for the non spaced colon, but I can't stand it. :)}

\mathbf is the math access for the text bold font.

If you load bm package after the euler fonts are set up you can use \bm as shown above .

\documentclass[%
    10pt,%
    a4paper,%
    onside,%
    headinclude,%
    footinclude,%
    BCOR5mm,%
    captions=tableheading]%
        {scrartcl}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage[french]{babel}


\usepackage{amsmath}

\renewcommand{\vec}{\mathbf}

\usepackage[%
    nochapters,%
    beramono,%
    eulermath,%
    pdfspacing,%
    listings]%
        {classicthesis}

\usepackage{bm}
\author{Author}
\title{Test}
\date{}

\begin{document}

\maketitle



As you can see, there is a big difference:
\[
   a \neq \vec{a} \bm{a}
\]

\end{document}