[Update: A patch has been deployed that aims to fix some of these issues.]
An inquisitive player has stumbled across some frame-rate weirdness in the just-released PC version of Vanquish, and if the game doesn't feel quite the way you remember it, this issue could be the culprit.
NeoGAF user .:Wesker:. brought comparison gifs that better demonstrate the problem but, in short, you'll take more damage when running Vanquish at higher frame rates. We've seen similar oddities crop up in PC ports of titles like Dark Souls II, which for the longest time tied weapon durability to frame rate, forcing certain players to repair their gear far more frequently than ever intended.
As explained by .:Wesker:., "If you lock the framerate to 30FPS AR mode will activate after 6-7 seconds of taking enemy fire but if you're playing at a higher framerate, like 60FPS, it will activate in just 1-2 seconds."
You're potentially in for a steep challenge, then. Until this is resolved -- either by Little Stone Software and Platinum, or potentially by a kindhearted modder -- you may want to keep Vanquish at 30FPS.