It's normal to have fluorescent yellow urine while taking vitamin B. B vitamins are water soluble. The vitamin molecules themselves are yellow and bodies just flush out extra dosages of them so what your body does not use it gets washed out. No harm is being done to your body
One thing, not drinking enough water can also cause the urine to turn yellow, so you could be having a combination effect with the B vitamin and dehydration. I assume that you are drinking plenty of water, but it is something for all of us to remember, especially since if one is taking a B vitamin but not drinking enough water, the B-vitamin link to the yellow color might be a fakeout.