When there is no one occupying the vehicle (that has Fire from Vehicle capability) other than the player, the player is able to switch seats. If there is an AI or another player occupying the driver/passenger seat, the player can no longer switch seats.
With launchers, they cannot be equipped when firing from vehicle because of back blast. Although the launcher's back blast doesn't to seem to have any effect upon players and vehicles. It would be good if the player can fire launchers from the vehicle but they have to ensure that the back of the launcher isn't facing the vehicle otherwise they will blow themselves up.