Theoretically, if vampires eat something, they can pee it out. They have to store the blood they take in somewhere (their digestive tract), and one could wonder if they actually use up every single last scrap and cell from the blood they ingest. No one has stated if vampires actually need the iron, the hemoglobin, the cell walls, etc. from the blood they get, or if they just need the life force in the blood.

Also, if vampires can cry blood (usually accepted as what they cry if they cry), then they can probably pee as well. They have internal systems that aren't normally used, but plainly do get some use from time to time. It'd be bloody, which is gross, but that's besides the point.
 
    

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