\feynmandiagram [inline=(d.base), horizontal=b to d] {
    a [particle=$b$] -- [anti charged boson] b [dot, label=left:$V_0$] -- [fermion] c [particle=$c$],
    b -- [anti charged scalar] d [particle=$f$],
};
+
\feynmandiagram [inline=(d.base), horizontal=d to b] {
    a [particle=$b$] -- [charged boson] b [dot, label=right:$V_0^*$] -- [anti fermion] c [particle=$c$],
    b -- [charged scalar] d [particle=$f$],
};