I did some trials to try to work out why my fps in VR was so variable - just exiting VR and re-entering VR it could change by 10 fps (both up and down). From the previous experience using DX12 in FS2020, I obviously suspected VRAM problems:
It really does look like there is a VRAM memory leak - that is, things are being stored in VRAM, but not removed when no longer needed. Therefore VRAM usage depends on what you have done in the sim since you last started the sim.
As I (almost) always run in VR, I enter VR mode as soon as I can (after pressing the initial Start button), as I do not want to fill the VRAM with 2D data which will not be needed. I will not exit to 2D mode till I am exiting the sim. Doing this, my VRAM usage stays below 8GB which allows a bit of contingency with my 11GB card. My fps stays near the top level of my expectations (around 30 on my system) and never drops to single figures (or even the teens) as it used to. I will avoid doing multiple flights in one sitting.
For those having trouble in 2D as well as VR, it may be advisable not to do too much jumping around from one flight to another without restarting the sim, to clear the VRAM.