I already provided one perspective here, but I would like to point out an additional major problem of re-using the vault unlock PIN as the user verification PIN:
By design, the vault unlock PIN is not synced between devices, which makes sense — security requirements for a desktop PC with restricted access in a locked office are going to be very different from the security requirements for a laptop or mobile device, which would have higher risk of theft or loss.
In contrast, the most logical and user-friendly implementation of User Verification PINs is that the UV PIN should be synced, so that the PIN that is used for performing the UV gesture during a passkey authentication ceremony will be consistent no matter which device is being used.
Again, this is yet another example of the fact that the requirements for a vault unlock PIN are fundamentally different from the requirements of a passkey UV PIN, and that confounding the two would be a flawed design.
Perhaps you could give users an option to use a single PIN for both functions, but if so, it would be essential for users to be able to disable such an option and keep the two PINs decoupled (for reasons explained here and in my previous response).
I would also like to point out that the ability to sync UV PINs is more of a “wish-list” feature that could be developed and implemented at some point in the future, as long as the near-term implementation of UV PINs does adhere to the premise advocated here — i.e., keeping the UV PIN as a separate PIN, independent of the vault unlock PIN.
Finally, I would like to point to this exchange that I recently had with @Micah_Edelblut. The upshot is that if a separate UV PIN is implemented (which is essentially what is being requested in this thread), then that new PIN could be leveraged to fulfill other functions unrelated to vault unlocking — for example, it could also be used as a PIN for protecting individual vault items (as an alternative to master password re-prompt).