What a terrible time your little guy must be having! I would probably take him in at this point, maybe just to have him checked, but as long as he's not running a high fever or seeming dedydrated (you said lots of wet diapers still), I wouldn't worry much.
Just a thought about the vomiting: Every time I call our doctor about vomiting (which I probaby do every time one of my kids throws up; I get a little nervous), they tell us no solid foods for about 24 hours after they vomit. Stick with water (or Pedialyte if he's keeping it down), maybe diluted juice, and breastmilk or formula. I would stay away from cow's milk too, if he's drinking it yet, just because on a very upset stomach, it can be irritating. Just try to keep him hydrated a little bit at a time. If he drinks and then vomits, then start giving him small amounts (like an ounce) at a time, wait an hour or so, then another ounce. It may help him to keep things down if there's not much for his little tummy to deal with at a time.
Oh, and for diaper rash, I like Desitin Creamy, which worked wonders for a terrible rash overnight on my middle one. I also like Butt Paste, since Desitin usually irritates my kids' skin and Butt Paste is extremely gentle.
Hope he's feeling better soon!