\feynmandiagram[baseline=(a.base)] {a[crossed dot]};
=
\feynmandiagram[baseline=(a.base)] {a[empty dot]};
+
\feynmandiagram [small, baseline=(b.base), layered layout, horizontal=i1 to a] {
% Draw the top and bottom lines
i1[dot, label=$V_0$] []
-- [double charged boson, edge label=$b$] a[dot,label=$V_0^*$]
-- [double charged scalar, edge label=$f_\sigma$, bend left] b[crossed dot],
i2[dot, label=below:$V_0$] []
-- [double anti charged scalar, edge label'=$f_\sigma$] c[dot,label=below:$V_0^*$]
-- [double anti charged boson, edge label'=$b$, bend right] b,
% Draw the two internal fermion lines
{ i1 -- [double fermion, edge label'=$c_\sigma$, bend left] c },
{ i2 -- [double fermion, edge label'=$c_\sigma$, bend right] a},
};
+\,...