I'm sure you are aware that the uretheral tube that leads to the bladder is not connected to the vaginal canal, though it could be that using something internally would cause bacteria from the outside to be pushed up the urethra. However, if this is the case, she would probably notice issues after sex as well. My suggestion would be to use only silicone toys and sterilize them between uses to ensure no cross contamination and (she should always do this regardless) make sure she urinates immediately after sex or masturbation to remove any bacteria that may have become pushed up into her urethral tract.

Also, on rereading your post I noticed that you said the infection always shows up the day after using a vibe. Most likely this is nothing more than a coincidence as it would likely take longer for a full UTI to develope. It could be that using the vibe, and the resulting contractions during orgasm, irritate the bladder and make the already present infection feel more painful? Or maybe they aren't UTI's at all, but some sort of allergic reaction that is being misdiagnosed? She should probably see her OB/GYN for more information and an exam.