Can well-cleaned sex toys used only vaginally cause yeast or bladder infections?

They should not: The key to your question is "well cleaned." if they are indeed sterilized after each use and well cleaned, then an infection should not occur.

Cause or predispose: To be specific, a toy without germs will not bring them . However, the introduction of a sterile toy to a region of the body that would secrete sexual fluids plus the mechanical action involved in masturbation would recruit nearby skin/vaginal germs. Many women have an increase in UTI's with the mechanics of intercourse & toys would increase risk the same way.