In a question-and-answer session with the Dio Message Board, former DIO keyboardist Claude Schnell discussed the often discussed and controversial exit of guitarist Vivian Campbell in 1986, "Prior to Vivian leaving, there was a slow and steady, yet very perceptible sense that he was drifting away from us," Claude explained. "As I recall, it was genuinely bizarre. It first began with him sporting a quite distinctive haircut which, in and of itself, was unremarkable. But then there were more and more little tell-tale signs that things just weren't right, like the ever-present Walkman headphones he would wear on the bus. Again, not a big deal, but as a result he would hardly ever engage in conversation with anyone. It was like he was off in his own little world. And little by little, slowly but surely, he just wasn't 'there' anymore. In retrospect, he had very strong feelings about how he was being, and had been treated; and I think his actions were largely predicated upon those feelings. Once Vivian was gone though, it seemed like [guitarist] Craig [Goldy] magically appeared out of nowhere, almost as if the switch had been carefully orchestrated. Not that I would presume to suggest such a thing, but at the time it was a widely held impression."
Campbell has gone on record saying he wasn't paid as much as he'd expected and that he strongly disgreed with Ronnie James Dio's band leadership style. Dio has generally not commented on the specifics in recent years, but has tended to be respectful of the guitarist when doing interviews.
Read the entire interview at the Dio Message Board under the "Don't Talk To Strangers" section.