This isn't documented anywhere, but that "attach to TBox" button is a remnant of an earlier feature designed for local testing and ingest work, not for production deployment of an ontology. Enabling it causes access to the entire TBox to become vastly more inefficient. We'll plan to remove it in 1.6 because it no longer serves an important purpose.
What you probably want to do is add the VSTO file(s) to the /WEB-INF/filegraph/tbox directory and restart Tomcat. Each file will be loaded into its own separate graph, and if you change the files on the filesystem and restart, the triple store will be automatically updated. You should see no performance hit using this method; in fact you'll notice this is how the main VIVO ontology itself is loaded.