What is cross-immunoreactivity, and how does it impact vaccine development?
Cross-immunoreactivity is just as you stated where an antibody that is specific to one antigen may recognize another antigen. I don't think this term is widely used as I see cross-reactive or broadly reactive more often. With regards to vaccines against viruses, cross-reactive immune responses are very important to establish an immune response that is effective against varying strains. Using Influenza as an example, an antibody that recognizes the HA protein of the H3N2 strain and also recognizes the HA protein of the H1N1 strain would be cross-reactive. If we had a vaccine capable of this we would not have seasonal flu.

Epitopes are specific regions on the antigen that are recognized by T cells or antibodies. They are the specific area on the protein structure that is recognized. Thus, for a response to be cross-reactive that specific structure must be present on both antigens. Epitopes could be located in any region of the protein, viruses will often have a region that is highly antigenic and highly variable to trick the immune response into focusing on the highly variable region rather than the more conserved regions.

Antigenic convergence is related to the sequence space available to the virus. Since viruses have limited space  for genes the amount of possible variation is limited as a single point mutation can have a large effect on the virus. Taking this into account some predictions could be made on how the virus might evolve. If the virus is evolving in such a way that there might be conserved epitopes, those epitopes might be important for cross-reactivity. Here is a paper on antigenic convergence in Hepatitis C. http://www.nature.com/articles/srep00267

This is a paper focused on flu, but it outlines the ideas surrounding cross-reactive antibody responses.
http://www.ncbi.nlm.nih.gov/pubmed/26175732