I just tested VK_EXT_memory_budget with driver 425.30 on an RTX 2070 and Win 7, and the budget for the device memory is too high. If i try to allocate 95% of the budget minus the usage, which i should be allowed to do, i get an out of device memory error.
The reported budget is the entire VRAM, regardless of OS or other application usage. The usage only reflects the VRAM that my process uses, not the entire system. Like this, the extension is useless since it cannot be used to control memory usage. Based on the spec i believe the budget is too high and should reflect VRAM reserved/used by the OS and other running apps.