It sounds like you are taking the players head position into account when setting the pose, similar to how the template teleport function works.
Generally you’ll want to set the camera based pose to the intended view at the start, and after that only moving the the entire player pawn (or the VR origin component if both the player and vehicle are in the same actor), not accounting for the camera pose. You can visualize this as attaching the players room to the vehicle instead of their head.
An easy way of setting it up is parenting the player to the vehicle, instead of setting the pose every frame.