How likely is it for a yeast infection get so bad that it could cause some vaginal bleeding?

Yes: If there is enough irritation from the infection, it can cause breakdown of the skin and bleeding. Also, if you are scratching or rubbing the area because of itching, this too can break down the skin and cause bleeding. Finally, some preservatives in some of the treatment creams might also irritate - if this is case, talk to doc about alternative treatment such as diflucan (fluconazole).