We examine the growth of structure in three different cosmological models with interacting dark matter and vacuum energy. We consider the case of geodesic dark matter with zero sound speed, where the relativistic growing mode in comoving-synchronous gauge coincides with the Newtonian growing mode at first order in $Lambda$CDM. We study corrections to the linearly growing mode in the presence of interactions and the linear matter growth rate, $f_1$, contrasting this with the velocity divergence, $f_{rsd}sigma_8$, observed through redshift-space distortions. We then derive second-order density perturbations in these interacting models. We identify the reduced bispectrum that corresponds to the non-linear growth of structure and show how the shape of the bispectrum is altered by energy transfer to or from the vacuum. Thus the bispectrum, or higher-order correlators, might in future be used to identify dark matter interactions.
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