\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning} 


\begin{document}
\begin{tikzpicture}
    \tikzset{
        %Define standard arrow tip
        >=stealth',
        % Define arrow style
        pil/.style={->,thick}
    }

    \draw[fill=green!30] (0,0) -- (90:.75cm) arc (90:27:.75cm);
    \draw[pil,color=black] (0,0) -- node[right=2pt] {$\vec a$} (27:2.2cm);
    \draw[pil,color=black] (0,0) -- node[near end, right=-3pt] {$\vec b$} (90:2cm);
    \draw(60:0.5cm) node {$\varphi$};
  \end{tikzpicture}
\end{document}