My neighbor growwing up had a 784, basically one size bigger then the 684. It was a nice utility type tractor and was his main tractor. With that said, i'm afraid you might be a little lacking in power/weight with the 684 and the 504 (4x5ft bale) vermeer. I think the 684 is prob. about 60 horse, and I think you would like at least 80 horse for the baler. It does depend if you are in hill country too. The 684 will prob work, but I just feel you might want to go a lil bigger on tractor, just my opinion anyway. We run a 140 horse on a 5x6 deere baler and in our mountains I wouldnt want any less tractor.
Jenna