Ummm, Voodoo was written by Paul Rodgers, it's totally his style. I don't think it's similar to Rock In Rio, except the fact that both are blues songs (which is not much of a similarity though).
i am pretty sure that voodoo is an older paul rodgers composition. the reason for this is that i saw qpr on the last 2 shows in london and i could see the teleprompter and it had no lyrics on it while voodoo was played. paul sang it flawless. another song without lyrics was his own old composition seagull. so if paul can not sing any of the queensongs without looking on the teleprompter now and then ( for what i don't blame him at all ) it is pretty obvious that he has the lyrics from voodoo in mind for a long time. anyway, unlike most other fans i met, i realy love voodoo and for me it is one of the coolest song from tcr. it has a kind of dire straits "brothers in arms" feeling to it and to be honest, who ever expected them to play it live? it was a real surprise to me.