You probably share my pain. For some dull reason Bethesda decided not to add any first person models for the sleeveless variant of dawnguard armor, resulting in annoying seams in front of your face every time you decide to look downwards, it's especially bad with Comprehensive First Person Animation Overhaul mod. So I decided to fix that.
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The solution was fairly straightforward, I just assigned some appropriate looking sleeveless beggar's clothing first person model instead, and that pretty much did the trick.
So normally all armors in the game have their respective armor addons in the plugin, and among the properties we've got the first person model for body armor, in this case Bethesda clearly missed, and forgot to add any one for this armor, and the game shows you some broken distorted third person model instead, which is especially noticeable with a mod like Comprehensive First Person Animation Overhaul, where you constantly see this shit flickering in your face whenever you move. I haven't really found the dedicated first person model for Sleeveless Dawnguard Armor, but I decided to find something that looks close enough in First Person, so Beggar's Clothing was my choice.
Recommended: Knight Armor - Ebony Edit - Fixes a similar issue you may encounter with Ebony Armor using some mods.
The Files section contains normal vanilla looking sleeveless model. Also contains a sleeved version, compatible with Sleeves of Skyrim.