Technically it is not a bug as the bosses you mentioned are considered vehicles and not a particular enemy type. If you ever have a bounty or quest asking for a Vex strike boss then just run the Pyramidion strike directly from the Director. That boss will count as it is not a vehicle.
That's the technical explanation and I already said that I have to run Pyramidion, so you're not contributing any new information at all.
What I am calling a bug is an unintended game behavior. We can agree that Dendron, Root Mind is a Vex, right? Whether or not it is a vehicle, toilet, building - it is a boss that is Vex. Therefore, by all logic provided by the game, defeating this boss ought to count towards an objective that asks that Vex bosses be defeated.
Do you understand?