The piezo pickup on each string allows a profile of various instruments to be superimposed. This works really nicely with very little latency and keeping a lot of the dynamics and resonance of the modelled guitar. It all hinges on how well the profiles are taken, for example the Stingray and Rickenbacker models on the VB-99 pretty close, you'd be hard pressed to tell the difference blindfolded. However for me the P basses on both units are a long way off.
Where the VB system really shines is that you can assign 2 separate signal paths (with 2 different modelled basses, synths, upright or "guitar"). You can then independently pitch shift or add effects to either path. The poly effects in the VB allow you to assign different effects to each string! Add to that the amount of assigns you can attach to either a footpedal or the controllers on the unit and it becomes a very powerful tool. It's a really well thought out and incredibly deep system once you start to delve, but not without its faults.