A better frame rate is supposed to make games easier, not harder. So when players discovered enemies were doing more damage at higher frame rates, it was inevitable that the developers would respond.
The bug, as first outlined on NeoGAF, would result in enemies doing more damage if your frame rate was above 30fps. It wasn't an immaterial bug, either: the more damage you took, the faster your game dipped into "oh shit" slo-mo.
It's nice for those who enjoy an arbitrary challenge with their video games, but it's certainly not what the devs intended. And in an update to a Steam announcement early this morning, SEGA has announced that a beta fix is available if you'd rather play the game without the FPS handicap.
To install, go to Vanquish in your Steam library, right click and select “Properties,” then go to the “Betas” tab. In the field to enter the beta access code, type the password
(caps-sensitive); “public_patch” will now appear in the drop down menu. Select it and close the Properties menu.
The patch, which SEGA stresses hasn't gone through QA - it's a public holiday in the United Kingdom right now, as well as the United States - corrects problems with skipping cutscenes, options resetting to default, health regen at higher frame rates, and invincibility frame timing (the issue that resulted in the FPS bug above). The beta patch also adds the "-monitor x" launch option, where x can be replaced by a number representing another screen for those with multi-monitor setups.
It comes a few days after launch options were added for an adjustable field of view (between 40 and 120 degrees) and custom aspect ratios. Not as nice as having a slider in-game you can tweak, but certainly better than not having it at all.