We have identified the issue that led to this fork.
It turns out there is a slight difference in timestamp validations between v2 and v3 (mainly due to their different codebases), which in the end makes v2 reject a few rare instances of v3 blocks.
Vault v2's refusal to incorporate a single of those v3 blocks, at the same time that all Vaults v3 consider it good, ended up creating a fork.

We are currently working to fix this issue in v3 alpha ASAP. In the meantime, we recommend not using Vault v3 as it could end up in a fork, and you will have wasted all your staking time while in that fork.
Just to be clear, the fork does not affect Vault v2, and there is currently no problem in running it.