http://www.tapdancinggoats.com/bold-vectors-in-latex.htm

Vectors can be typeset in LaTeX with the command \vec, which decorates the argument with a little arrow. This was cute at first, but it doesn’t look very good, especially in fractions. Textbooks use bold face for vectors, so here’s how to do that in LaTeX.

\let\oldhat\hat
\renewcommand{\vec}[1]{\mathbf{#1}}
\renewcommand{\hat}[1]{\oldhat{\mathbf{#1}}}

This also makes unit vectors (typeset with \hat) bold.