The antibodies from BF are IgA and do not interfer with
vaccines. Mom's IgA will enter the child's GI tract and
possibly upper respiratory tract. It doesnt get into the
general circulation.
Maternal antibody passed in utero is IgG and enter
baby's circulation.
There was a measles vaccine given at 8 months but
maternal antibody interfered with it at this age. In the US at
least this vaccine was discontinued in the around late 1980's
or early 90's. A little before my time.  OK no need for the Kathy
D and Linda Smith style "Ohmygawd youre just a baby" and
"help me across the street some time" comments.
Breastfeeding enhances the responce to vaccines not
interferes with them.
For those with vaccine questions Id recommend "What
Every Parent Should Know About Vaccines" by Paul Offit MD
and Louis Bell, MD.
The US CDC has an Email address to "ask the experts
about vaccines. I dont have it handy but will find it.
-Rob
"In the face of such diseases, the most dangerous experiment
of all is to do nothing." - Joseph Stokes, Jr, MD (1896-1972)
in NEJM,1967, in reference to deaths caused by measles and
mumps virus