Are vaccinated dogs still carriers of diseases and infections? I.e. can they pass things on to unvaccinated dogs even though they are vaccinated? This is just a theoretical question not related to anything I am experiencing - I am just very bored and feeing quite wistful in the face of having to write a PhD thesis..!

Yes they are still carriers as are we. Especially withthings like Parvo etc.  which we can carry on our shoes etc.