The idea is nice,but I have to agree that a prequel wouldn't work (in my opinion).The only thing I can think of is setting a GB movie in between the first and the second,but I very much doubt that would work either (it would probably destroy the franchise aswell).I think its best to stick with the idea of a second sequel.

how bougt the story behind vigo or even better because of the giant sloar GOZER

The story behind Vigo could be quite interesting,but it wouldn't be a prequel,it would be more of a spin-off (just like The Scorpion King movie will be to The Mummy Returns).We need to see whether The Scorpion King pulls it off first.But I do like that idea.