Right now spigot does not call the PlayerTeleportEvent/EntityTeleportEvent for entities that are being used as vehicles.
This is, judging from https://hub.spigotmc.org/jira/browse/SPIGOT-2064 this is intended behaviour and makes sense as the entity does not teleport at all. This begs the question tho on how to effectively register logic for the specific edge case of a plugin attempting to teleport a entity that currently passengers on them.
An example usecase of this is the usage of passanger based pets etc that are supposed to follow you through teleports attempted by other plugins.
This issue suggests the implementation of a specific event that will be called prior to the spigot specific teleportation such as a PreEntityAPITeleportationEvent. Notably I do understand that this event would not match any vanilla event and would therefore purely represent spigot mechanics, hence I would like to at least foster a discussion about this issue and explore other potential solutions to it in this issue if my initial suggestion is deemed incorrect.