If you are ready to merge repository A into MVC, then you can go ahead and do so. Then you can merge MVC into B, at which point B will have the bugfix as well. The red book has a good tutorial of merging if you need one.
If A isn't ready (and this sounds like your situation), it's a little trickier but not impossible. What you can then do is transplant the change from A to MVC. Transplanting is basically a way of duplicating a changeset onto a different parent node. (So if changeset Z had parent X but you wanted to apply it to parent Y, transplant's your best friend.) The Mercurial transplant extension documentation has more information and a few helpful diagrams.