Mass grave in London reveals how volcano caused gl

If true presumably the effects wouldn't have been restricted to London, I wonder if there is any records from Paris or other cities of the time.

Unfortunately, this doesn't let the fleas and rats "off the hook", because of the vector by which plague is spread - i.e., hosted by rats and spread by the bite of their infected fleas.

A natural disaster would certainly make the population more vulnerable to disease, but would not change the natural vector of the disease.  If anything, a volcano would have made the rats more anxious to move around and circulate through the population, thus further aidng the spread of the plague.