Hello! My name is XXXXX XXXXX it will be my pleasure to help you with your dog today.
As you may or may not know, Adderall is actually a combination of four different stimulant medications (amphetamines). According to all the literature I've seen, you would start to see signs of Adderall toxicity at around 0.5mg/kg. So if your dog who weighed 42 pounds, a toxic dose would be approximately 9.5mgs, I'm not sure how much your dog weighs, but it sounds like he did only get a very small amount.
At this point, I don't think I'd be too concerned about it. You can keep an eye open for signs of toxicity such as hyperactivity, racing heart rate, panting or agitation and restlessness. If you see any of these signs, you need to have him seen at an emergency vet immediately. They'll get him started on IV fluids to flush the system and activated charcoal to bind to the ingested medication...however, as I said, the small amount he got probably won't cause those signs.
As for the vomiting...it's actually very, very common for a dog to continue to vomit in these cases. The best thing you can do is to limit the amount of water he gets and don't feed him. This will just get his stomach re-upset. Allowing the stomach to rest by not feeding/watering him until morning will get that vomiting to stop.
I hope this helps.