Hi Richardh,
I've got a Vault and a Zest, so have been able to compare them, albeit briefly, back to back. I've been paddling since April last year, so I'm no expert by any means, but I do about 50km per week on average. My paddling waters are a mixture of flat and ocean, with rather small, short wavelength swells. I did a back to back paddle in about 8-10 knots of breeze, around a man-made island just offshore, so the conditions were extremely mild. The loop is just over 4km, starting with an upwind paddle in flat water, then around the top of the island with confused chop coming from three directions, then an angled downwind run with the swell coming over my left shoulder, finishing off with a sheltered paddle in the lee of the island, and across flat water back to the start. By no means difficult or testing conditions, but it is a start. Compared to the Vault, the Zest is very stable. My girlfriend, who can't stay upright in the Vault for more than 10 seconds, on her second paddle of the Zest managed 1.5km without falling out. In the confused chop at the top of the island I had to concentrate hard with the Vault, but hardly gave it a thought in the Zest. The bucket is a bit wider on the Zest (I think), and it makes dropping into the seat during a remount a lot easier. Remounting in general is a breeze. Despite its stability it doesn't give much, if anything, away in terms of speed as far as I've been able to ascertain. My times round the island were within a few seconds of each other, but I had to stop three times on both skis to clear weed from the rudder. Manoeverability is good, with the ski responding well to small rudder inputs. Without measuring, it seems to have a bit more volume in the nose than the Vault, which if it has could be a benefit on downwinders, but might then make it more affected by cross winds. Due to the low wind and small swell the downwinder had nothing to offer, except for one run that I caught, surprisingly on the Zest. That's about all I have to offer for now on the Zest, and won't be able to compare it in more extreme conditions until the South Easter starts blowing again in April.
Regards, David