How do I force-indicate a vocabulary as a source for node breadcrumbs?
You can try Crumbs instead of taxonomy breadcrumb.
You can be bold and try the 7.x-2.x branch (*)

Documentation for Crumbs + Taxonomy: http://drupal.org/node/1984484

I have not personally tried it in combination with taxonomy menu, but think it should work ok.

To solve the problem on node pages:


Install Crumbs
Visit Admin > Structure > Crumbs to adjust the plugin priorities (*).
The taxonomy.termReference.field_vocab_B.node.certain_node_type (**) needs to be moved up, into the "Enabled" section.


You can do more stuff with Crumbs (you could even replace taxonomy menu with menupoly), but one question at a time.

(*) The crumbs-7.x-2.x, among other things, has a revamped admin UI to set the plugin priorities. It is still not perfect, which is the main reason there is no 7.x-2.0 stable yet.

(**) Thanks to the wildcard system, you could also move taxonomy.termReference.field_vocab_B.node.* or even taxonomy.termReference.field_vocab_B.* into "Enabled", instead of taxonomy.termReference.field_vocab_B.node.certain_node_type. Doing that would activate this behavior for other node types and/or other entity types that have field_vocab_B.