I always recommend an impact vest for beginner hydrofoilers. Crashes are a lot harder when they come from one meter up. After a while you learn how to fall and the crashes don't seem to hurt as much. But as you progress you will find yourself going a lot faster sometimes..
Personally I have been wearing an impact vest for several years now. It provides some floatation and noticable warmth. It is just part of my regular equipment. I find the Oneill pull over model comfortable and non intrusive.
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I don't think an impact vest/lifejacket or helmet are necessary for other aspects of kiting (apart from obvious exceptions like hitting sliders/jumping rocks/kiting well offshore), but for foiling there are so many sharp parts and things that can very easily go wrong that I think it is probably worth it. I've almost taken a foil to the ribs a couple times when jackknifing, luckily avoided it every time so far but I could easily imagine getting speared by it. I don't really care about the flotation (I have a kite for that) and the warmth is a negative here but I think the actual protection it offers from the foil is a good idea.
I use NP Surf Rise vest always when I go foiling and always when I ride deep water. Two reasons:
1. Floatation assistance. Jimbo told his story of having a cramp and almost drowning. Floatation vest is my extra bit of floatation, not sufficient nor certified though.
2. Extra cushion in case I get hit by the foil. I hope it might save me a rib.
During learning on faster foils makes a huge difference in getting the wind knocked out of you. I continue to wear it for flotation due to foil kite self rescues in light wind....so much easier with a bit of float to help.
I've never used a impact vest since I started foiling. I started using a helmet after having a few face plants when I first began foiling. Now that I have upgraded from a LF foil fish to a Moses fluente my speed has increased dramatically so I'm reconsidering the impact vest but being a wave rider I don't know if I really need it. Less is more when it comes to wave riding.
I don't think impact vests make a lot of difference in terms of hitting the water. If anything they make it worse because there's a bit more drag when you hit.
I think the intended purpose of an impact vest is for protection when hitting solid objects such as boat rigging and sliders and stuff.
Same goes for helmets. They don't protect you much from water impact. They do offer some protection against hitting hard objects.
You could make a case of wearing a vest for protection against impact with the foil or board. I have found that such impacts are quite rare and the consequences a lot less severe than people imagine. One thing that has helped is using a lightweight carbon board and foil. It's lighter so it bounces off and the bumps are a lot more gentle.
I do wear a buoyancy vest, but that's mainly to be seen to be legal and to be a bit sensible. We tend to go faster and further when foiling so there's more chance of needing a rescue further out to sea.