\begin{tikzpicture}[scale=1.5]
  \def\r{1}

  % origin
  \node[circle] (O) at (-2,-2) {$\vec{0}$};

  % circles and their centers
  \node[scale=0.01] (D1) at (\r,0) {$\cdot$};
  \node[scale=0.01] (D2) at (0,\r) {$\cdot$};
  \node[scale=0.01] (D3) at ($(D1)!0.5!(D2)$) {};
  \draw (D1) circle (2*\r cm);
  \draw (D2) circle (2*\r cm);
  \draw[->] (O) -- node[below, sloped, near start] {$\vec{z}_1$} (D1);
  \draw[->] (O) -- node[above, sloped, near start] {$\vec{z}_2$} (D2);
  % \draw[->] (O) -- (D3);
  \draw[->] (O) -- node[near end, above, sloped] {$\frac 12 (\vec{z}_1+\vec{z}_2)$} (D3);

  % The two copies of a
  \draw (D1) -- node[near start, above, sloped] {$a$} (D2);
  \draw (D1) -- node[near end, above, sloped] {$a$} (D2);

  % The triangle's other leg
  \node[scale=0.01] (X) at ($ 1.83*({\r},{\r}) $) {};
  \node[scale=0.1] (D3) at ($(D1)!0.5!(D2)$) {}; % center of new circle

  % triangle's two hypotenuses
  \draw (D2) -- node[above, sloped] {$r$} (X);
  % \draw (D1) -- node[below right] {$r$} (X);
  \draw (D3) -- node[midway, sloped, below] {$\sqrt{r^2-a^2}$} (X);

  % the new, smaller circle
  \draw[dashed] (D3) circle (1.85*\r);

  % some points
  % \node[scale=2] at (0.5,1.5) {$\cdot$};
  % \node[scale=2] at (0.3,-0.2) {$\cdot$};
  % \node[scale=2] at (0.2,1.0) {$\cdot$};
  % \node[scale=2] at (1.4,0.8) {$\cdot$};
  % \node[scale=2] at (0.7,-0.4) {$\cdot$};
  % \node[scale=2] at (-0.4,0.1) {$\cdot$};
\end{tikzpicture}