Monogamous prairie voles are known to have higher levels of receptors for these neurotransmitters than do voles who have yet to mate; and when otherwise promiscuous montane voles (M. montanus) are dosed with oxytocin and vasopressin, they adopt the monogamous behaviour of their prairie cousins.
To make these promiscuous voles truly monogamous though the drug by itself won’t do all the required molecular changes - the context is also needed. It was in the voles that were subjected to 6 hours of cohabitation prior to the injection where the changes were seen. The epigenetic change appears to be necessary for the long-term change in behaviour.