I want to present an experimental idea for this.
Since you are using VR, might it be possible to use some blurring/focal techniques like in cinema.
As an example, if you have a single lethal enemy among others, when that enemy enters the center field of view, blur the other lesser enemies and make the lethal enemy very clear with even maybe a small bit of zoom or enlarging. Perhaps increase the contrast in colors on that enemy specifically.
This might work best in stealth parts, where the player is likely to be taking their time and observing.
The reverse could work as well, depending on setting. If a lethal enemy is presented as some unknowable horror, or something beyond immediate comprehension, you could cause it to be harder to focus on. This simulates part of an adrenaline rush from panic where your pupils dilate and hence your vision blurs.
- Nausea: it's already an issue with VR, I'd suspect this wouldn't help unless the effect is balanced very well
- Timing and position: How long does a player have to have something in their field of view before this effect happens? Does it have to be in the center, or does it work at periphery as well?
- Tracking/Movement: Once the player looks away, does the effect persist? For how long?
- Gameplay Impedance: Will this affect prevent the player from interacting with other game elements?
Again, this is an experimental suggestion. I'm not aware of any games using this concept with VR. But I'd certainly like to see if it could work.