I cannot tell by everyone, but I do feel a tiny difference when hearing the 2nd and 3rd person imperative forms. To me 3rd person imperative sounds stronger. Maybe it's just because my mother used to ask me to do things using the 2nd person, while strangers prefered to use 3rd person.
In my region (Ceará), despite tha fact that we conjugate 2nd person the wrong way as most brazilians, there is still a little difference between tu and você. Você is informal, but tu is more.
<< More informal-------------------------------More formal >
TU ----- VOCÊ ----------------------------------- O SENHOR
But I think this is something that is fading away, and they are tending to have the same "weight". I would say most people wouldn't notice any difference between 2nd and 3rd person imperative forms and they are really interchangeable.
BTW, the conjugations are inverted, but again, nobody really cares/notices this difference in oral conversation. You just have to be more critical about that in writing.
This goes to show you that Brazil is a very large country and there are lots of regional differences. There are many places where you'd hear 'tu' being conjugated as 'você', and when it happens it's informal because they are conjugating it wrong, not because 'tu' is in itself more informal. In many places (if not most), you won't hear 'tu' at all. But remember that, to be formal, you should call people 'o senhor' or 'a senhora', like Wesley said.
"tu" is conjugated as 2nd person and "você" as third person, even though they both mean "you" (singular).
So would all combinations be normal/acceptable? (Você with vc conj, Você with tu conj, tu with tu conj, tu with vc conj)
It's informal. You should avoid it when writing since it is poor Portuguese and a grammar issue.
Given what Paulenrique and wesleyjefferson say about the equivalence of the "tu" and "você" imperative, I think it would be unlikely that someone would understand that meaning in an isolated sentence without the explicit use of the pronoun "ele". Of course, with more context it works: "Ele entra no quarto e acende a luz".