If you are looking for something to use on guitar and bass, the GR-55 will be the unit, with it's dual modes.
The VG-99 is only for guitar, and the
VB-99 is only for bass (usually).
The GR-55 modeling and effects section can be seen as half a VG-99 and half a
VB-99, with limited effect chain routing options,
plus two PCM synths (like having 2 GR-20's).
The GR-55 also has limited usable pedals, compared to the GT-10B or
VB/VG-99 with foot controller
(unless you add an extra midi foot controller).
For me, the GR-55 works very well, as Gastric explained, i use it for the odd bass models and mostly as re-enforcement for Rock type songs,
either as an effects only unit or syhth/pad type backing or song intros..
it has done well for live gigs.
Before i was using the GT-10B and a GR-20, and the GR-55 does the same + more,
and is more compact and light weight.
The other feature of the GR-55 i really like is the USB memory stick music file player,
I load it with all the music to practice with, and can quietly sit in the corner with headphones on,
only with bass and GR-55.