Just to clarify: the Vandals were not defeated by the Muslims. Their kingdom was destroyed by Roman general Belisarius in 533 AD, and the Vandals were removed from Africa and sent to Syria to fight Persians as a part of the Imperial army.

As for their appearance, according to Procopius who witnessed and described the fall of their kingdom, they still looked pretty "Germanic" - i.e. were tall, blond and with white skin.