\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{gensymb}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning,decorations.pathreplacing,shapes}
\tikzset{
>=stealth',
pil/.style={->,thick}
}
\begin{document}
\begin{tikzpicture}
\tikzset{
>=stealth',
pil/.style={->,thick}
}
\draw[pil] (0,0) -- node[near end, above] {$\vec a$} (3cm, 0cm);
\draw[pil] (0,0.5cm) -- node[near end, above] {$\vec b$} (2cm,0.5cm);
\end{tikzpicture}
\end{document}