After going through three VPS-100s, I'm using a Vivid Leak Sentinel V5XB with my housing - it's more expensive, but trouble-free and cheaper in the long run.I do wish they had put a leak detector in the case - having to unscrew it after testing and screw in the plug makes for 1 possible failure point, though it hasn't failed on me. (And the vacuum pump is pathetic - they sell a bigger one on AliExpress for US$10, as it turns out [not shown on their website], and an electric one for US$50.)
Can't comment on a6100/a6400, but a6700 autofocus is worlds better than a6300. The animal eye detection doesn't really work underwater, but general-purpose tracking AF-C is like magic - point the camera at a fish, hold down AF-ON, and it will snap a rectangle around it and won't let go, even with 90mm macro lens in fairly murky water.In terms of camera, maybe someone using the A6100/A6400 (and I think A6600/A6700) can chime in about how well the animal eye focus works for underwater subjects. The A6000/A6300/A6500 do not have it. (I'd like to know that answer, too, in case I ever upgrade.)