In brief: Not exactly. Blood actually doesn't get thinned.  what we call blood thinners are agents that decrease the ability of the blood to clot, usually used in people whose blood clots excessively.
Taking B-complex vitamins, which include vitamin B12, can cause urine to have a bright yellow or even orange color, but check with your doctor to be sure that's what is going on in your case. B vitamins are water-soluble vitamins, which means that what your body doesn't use is excreted in your urine. Riboflavin (vitamin B2) is especially likely to cause this color change in urine. Several medications can also turn urine a bright yellow or orange color. Changes in urine color may also signal certain health problems.
I would check and see about processed foods then, such as the Captain Crunch. If you are eating that much, then it's possible that you either are getting a bunch of Vit.B or you have a batch of something with a buttload of Vit. B, but it really sounds as if you have Vit. B in your urine. that is what makes urine bright neon yellow. Other processed foods also have added vitamins.