The easiest way to learn about arc
construction is to use the verbose mode at first. So the command
\draw [red, thick] (1.0,0) arc [start angle=0, end angle=30, radius=1cm]
node [midway, right] {$\phi$};
says to start the arc at (1.0,0)
with the provided parameters. A more concise way of saying that is to use
\draw [red, thick] (1.0,0) arc (0:30:1cm)
node [midway, right] {$\phi$};
where the parameter following the arc is (<start angle>:<end angle>:<radius>)
.

Code: Verbose
\documentclass{article}
\usepackage{tikz}
\usepackage{siunitx}
\begin{document}
\begin{tikzpicture}[>=latex]
\draw[style=help lines] (0,0) (3,2);
\coordinate (vec1) at (300:1.5);
\coordinate (vec2) at (30:2.5);
\coordinate (vec3) at (0:2.5);
\coordinate (vec4) at (90:2);
\coordinate (vec5) at (270:2);
\coordinate (vec6) at (180:2);
\draw[->,thick,black] (0,0) -- (vec1) node[right] {$I$};
\draw[->,thick,black] (0,0) -- (vec2) node[below right] {$V$};
\draw[->,thick,black] (0,0) -- (vec3) node [below] {$Re$};
\draw[->,thick,black] (0,0) -- (vec4) node [left] {$Im$};
\draw[->,thick,black] (0,0) -- (vec5);
\draw[->,thick,black] (0,0) -- (vec6);
\draw [red, thick] (1.0,0) arc [start angle=0, end angle=30, radius=1cm]
node [midway, right] {$\phi$};
\draw [blue, thick] (0.5,0) arc [start angle=0, end angle=-60, radius=0.5cm]
node [midway, right] {$\phi-\ang{90}$};
\end{tikzpicture}
\end{document}
Code: Concise:
\documentclass{article}
\usepackage{tikz}
\usepackage{siunitx}
\begin{document}
\begin{tikzpicture}[>=latex]
\draw[style=help lines] (0,0) (3,2);
\coordinate (vec1) at (300:1.5);
\coordinate (vec2) at (30:2.5);
\coordinate (vec3) at (0:2.5);
\coordinate (vec4) at (90:2);
\coordinate (vec5) at (270:2);
\coordinate (vec6) at (180:2);
\draw[->,thick,black] (0,0) -- (vec1) node[right] {$I$};
\draw[->,thick,black] (0,0) -- (vec2) node[below right] {$V$};
\draw[->,thick,black] (0,0) -- (vec3) node [below] {$Re$};
\draw[->,thick,black] (0,0) -- (vec4) node [left] {$Im$};
\draw[->,thick,black] (0,0) -- (vec5);
\draw[->,thick,black] (0,0) -- (vec6);
\draw [red, thick ] (1.0,0) arc (0:30:1cm) node [midway, right] {$\phi$};
\draw [blue, thick] (0.5,0) arc (0:-60:0.5cm) node [midway, right] {$\phi-\ang{90}$};
\end{tikzpicture}
\end{document}