I've tried both VHII and Nailbomb bridge in place of a Fred and can tell you that they are all quite different.
(I haven't had a PAF Pro so I can't comment on that)
To my ears the Fred has good mids, just enough bass and treble and overall fairly cohesive sound (some might prefer to say 'closed'). The VHII would be similar in output but it's much more open sounding, has more bass and highs too. The Nailbomb is much higher output and has a more aggressive high mid punch.
As Twinfan says, it depends what kind of upgrade you want: Do you want more power and aggression (Nailbomb)? or similar output but more space, air and some crunch (VHII)?