I think my 3 year old may have injested my little brothers juice with his vivance in it. He's throwing up, acting ocd, could it be something else?

Bring to ER. Ifma three year old ingest something he shouldn't have and his behavior is changed (neurologic / personality change), he must be seen urgently. At the er, they can contact the poison control center to make sure they are doing things right. The vomiting is good, since the toxic ingredients are getting out of his system. Just make sure he is sitting up and leaning forward so he doesn't aspirate.
Sounds like it! Those are symptoms of taking vyvanse (lisdexamfetamine) in a three year old! it should resolve in 8 - 12 hours and he should be back to normal. If not, have him seen by his pediatrician or family doctor.