The CPU is kind of like the brain of your computer. It handles everything that happens on your system, how the OS, software, devices and hardware components on the devices work. How much the game relies on the CPU as opposed to the VPU depends on how the game was coded, really.
Some games are coded to offload as much graphical information as possible to the video processor on the graphics card. What it can't offload it will still process on the CPU. And video games are very much about the graphics.
The CPU is still responsible for handling all the other instructions required to run the gameand still handle all the background functions of your PC that take place even while you are playing the game. If a processor can't digest all the requests that come in quickly enough, it will start to queue up requests and processor lag will occur.
The faster your processor speed, the more requests and interruptions it can handle before queuing and lag set in.
The best way to always tell is to find out the system requirements of the game. If you match the system requirements of the game to the PC, you should be fine. If you are below the requirements in just one area you may encounter instability and lagging/slowness.