While not being a full-blooded Disney villain as he never actually does anything particularly evil, Vincent is evidently a deeply disturbed boy and performs scientific experiments on the family dog, Abercrombie. Given the dog's menacing name, it's probably that Vincent named him. Vincent wants to turn Abercrombie into a zombie dog so that the two of them "can go searching for victims in the London fog", implying that Vincent plans to wreak great destruction.
It is probable, given his sadistic nature and his cruelty toward animals that he is a sociopath though he is most definitely a paranoid schizophrenic as he comes to the delusional belief that he is in fact Vincent Price, suggesting that he isn't particularly happy with the way he is himself and wants to become someone whom he admires, implying low self-esteem, something not particularly common among Disney villains who usually have decidedly inflated egos.
It is not particularly evident what was the catalyst of Vincent's mental illness as he apparently came from a loving family and a respectable home in a pleasant neighbourhood though reading the horror stories of Edgar Allen Poe and watching films starring Vincent Price evidently did nothing for his sanity or lack thereof.
He is aloof in mannerisms and bearing and carries a cigarette holder, a characteristic he shares with other Disney villains such as Cruella De Vil and Professor Ratigan. There doesn't seem to be a cigarette in the holder however as Vincent is evidently too young to smoke.
Unlike other Disney Villains, Vincent is shown to be artistic and smart, reading the books of Edgar Allen Poe and painting pictures of the wife he thinks he has.
But around other people, and when he is sane, he was a quiet, polite, considerate and nice young seven year old.
Vincent is a polite and intelligent young boy but is socially withdrawn and has a dark imagination. He imagines his aunt dripping in hot wax for his wax museum and has a macabre fascination with the works of Edgar Allen Poe. He eventually takes to the belief that he is Vincent Price and that his wife has been buried alive. After he digs up the flowerbed, thinking it to be his wife's grave, his parents realise that he is disturbed and confine him to his bedroom where Vincent thinks that he has been banished to the "Tower of Doom". After he is berated by his mother, Vincent finally goes completely mad and falls to the ground in a dead faint, quoting The Raven by Edgar Allen Poe before losing consciousness. At this point the film finishes so it is never clarified what became of Vincent but presumably he was comitted to a psychiatric institution.