Don't worry, your reflexes haven't slowed overnight – the PC version is actually harder than the designers intended it to be. An eagle-eyed NeoGaf user called .:Wesker:. seems to have has spotted a bug in the port that ties the amount of damage you take directly to your frame rate.
So, if you're playing on higher specs, or on lower settings, you're actually taking more damage than somebody whose game is running like a slideshow.
If fits in with what Alec said in his review, noting that he "died a lot in Vanquish (on normal difficulty)", which could become frustrating.
Take a look at the evidence below: the first gif shows the game at 30 FPS and the second at 60 FPS. Notice how quickly the screen turns red, which is an indication that you're nearly dead. Top sleuthing on .:Wesker:.'s part.
Bizarre, although not without precedent. A similar thing happened with weapon durability in Dark Souls 2 when it was ported to PC. The higher your frame rate, the quicker your weapons degraded.
That bug took a year to fix, but let's hope that Sega and porting house Little Stone Software can get Vanquish sorted a bit faster than that.
Vanquish is still worth playing, then, just maybe turn the difficulty down a notch. Or leave it where it is, you masochist.