Alignment of equations that contain Feynman diagrams

Use gathered:

\documentclass{article}

\usepackage{amsmath}
\usepackage{feynmp-auto} % or feynmf

\begin{document}

\begin{fmffile}{test}
\begin{equation}
\begin{gathered}
\begin{fmfgraph*}(65,50)
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmf{plain}{i1,v,o1}
\fmf{plain}{i2,v,o2}
\end{fmfgraph*}
\end{gathered}=-i\lambda
\end{equation}
\end{fmffile}

\end{document}