The monoclonal antibody trastuzumab and the EGFR/HER2 tyrosine kinase inhibitor lapatinib improve the clinical outcome of patients with HER2‐overexpressing breast cancer. However, the majority of metastatic cancers will eventually progress, suggesting the need for other therapies. Because HER2 overexpression persists, we hypothesized that the anti‐HER2 immune response induced by cancer vaccines would be an effective strategy for treating trastuzumab‐ and lapatinib‐refractory tumors. Furthermore, we hypothesized that the antibody response could synergize with lapatinib to enhance tumor inhibition. We developed a recombinant adenoviral vector expressing a kinase‐inactive HER2 (Ad‐HER2‐ki) to use as a cancer vaccine. Vaccine‐induced polyclonal HER2‐specific antiserum was analyzed for receptor internalization and signaling effects alone and in combination with lapatinib. Ad‐HER2‐ki vaccine‐induced potent T cell and antibody responses in mice and the vaccine‐induced polyclonal HER2‐specific antiserum mediated receptor internalization and degradation much more effectively than trastuzumab. Our in vitro studies demonstrated that HER2 vaccine‐induced antibodies effectively caused a decrease in HER2 expression, but when combined with lapatinib caused significant inhibition of HER2 signaling, decreased pERK and pAKT levels and reduced breast tumor cell proliferation. In addition, a known mechanism of resistance to lapatinib, induction of survivin, was inhibited. The combination of Ad‐HER2‐ki plus lapatinib also showed superior antitumor efficacy in vivo. Based on these results, we feel clinical studies using this approach to target HER2‐overexpressing breast cancer, including trastuzumab‐ and lapatinib‐resistant tumors is warranted.