Ideas
I have been experiencing the same issue since the launch, and it's not enjoyable. i7 13700k 4080 32 GB DDR5, not bleeding-edge equipment, but top-tier; no way in hell this game should be using 15.3GB of VRAM. It starts at nine, then game after game it creeps up to over 15 with significant degradation in FPS and quality, Hallmarks of Memory leaks.
I have been experiencing the exact same situation that you have described on an RTX 5080. (16GB VRAM) I am running the game on 4K with DLSS-4 Quality enabled. All of the graphics settings are set to overkill along with the texture quality setting. When i launch the game initially, the VRAM slider found in the graphics settings tab indicates a VRAM usage of approximately 9GBs. As i continue playing the conquest mode, regardless of the map, this value gradually increases and indicates that the game occupies 15.4 GB of VRAM. According to my observations so far, the longer i play, the more drastic this occupance becomes, which eventually causes hitches, stutters and unusually low fps counts (Drops from around 100-110 fps to 70s in my case.)
This condition perfectly fits the definition of a "VRAM Memory Leak" as you described. The community would be grateful if the developers addressed this issue.