So the amount of creatine changes the rate as which water is loaded into the cell, but the total amount of water needed is calculated based on your muscle mass. The figures I've seen is 2L extra water (spread evenly, not all at once) for a normally recommended 5g daily creatine intake during loading (which typically takes a month at 5g/day dosage).

You're fine if you take daily algae oil, multivitamin, iron supplement, calcium, and creatine (without going overboard on any of it) and making sure to get lots of protein from balanced sources (eating a ton of soy is actually not a bad idea, because it's a balanced protein), but I wouldn't recommend somebody who doesn't want to have to worry about their nutrition to go vegan. Enough omnivores are already deficient in vitamins and minerals, and veganism only makes it harder.

Today, because our food / water are so sterilized (good thing!), we don't typically get much bacterial B12 from unclean root vegetables, drinking from streams, etc. So yes, we need to supplement today. Because of advances in cleanliness.