Vulkan and DX12 are basically ways of optimizing CPU usage in graphics intensive applications.
This means that a typical power machine with an i7 and maybe even overclocked will see little benefit, since they are already not very constrained by CPU performance, but the benefit will increase on machines with a lesser CPU.
Reviews of graphics cards want to show the differences between graphics cards... so they try to minimize limitations in the rest of the system by using top end CPUs, fast RAM, etc... This amplifies any differences from the graphics cards, which is often what people want to see when they go to a graphics card review. I think people who are familiar with DX12 and Vulkan and what they do are not surprised to see a minimal benefit on most reviews (which typically use a 4 or 6 core i7), but a larger benefit on a low spec CPU. You are correct that reviews do not make this kind of thing obvious, but I don't think it's intentional... It's not an easy thing to convey that there's an interaction between the CPU and GPU and resolution. People want to think one dimensionally (GPU vs. GPU) rather than multi-dimensionally. The reality is that the system as a whole has to be considered.
In general, with a decent CPU, low resolution is more CPU limited (but usually at a minimum FPS number that is well over a threshold where you can tell a difference) and high resolution is more GPU limited. Since DX12 and Vulkan are essentially the same thing as using a better CPU, you see the benefit most at lower resolutions and on low spec CPUs.