Wow.. Nice job WM..
(why VSP's instead of azipods ? )
Much faster response; important when you have a heavy load dangling from a wire.
A VSP can change the direction of thrust almost instantaneously.
An AZ needs to rotate a pretty massive propeller assembly. It also takes time for the screw to change speed.
The VSP is always turning at constant speed. Only the phase of the blade angles changes.