I must create my own lab and I was going to use water, however, a similar experiment online says that acetic acid should be used to create my standard riboflavin (vitamin B2) solutions. Why is that?
The flavins are light sensitive and unstable in alkaline solutions. The dry solid is stable to diffuse light, but is highly photolabile in solution, especially in alkaline solutions. Neutral and acidic solutions are stable in the dark (3% decomposition per month at 27 °C at pH 6.0). Acidic solutions may be sterilized by autoclaving.
So making a slightly acidic solution probably helps to prevent decomposition. If you dissolve it in pure water without a buffer, the pH is not well defined.