I thought that animals couldn't enter city borders.
Having further thought this out, there would be a tech imbalance - i.e. it would take a few turns for the Vandals to find a city and overpower it. In the meantime, the other civs will have researched a couple of techs on top of their freebies. However, what I could do is make their starting techs Bronze Working (which is normally researchable after Tool Working, Mining and Warrior Code have been researched - but the Vandals would still have to research the other three techs to reap their individual benefits) and Boat Building, which normally requires a few other techs. That would give them a basic ship (in case they started the game on an island) and the spearman buildable after Bronze Working, which has 3 power against the 2 power that the basic Warrior has.
Having further thought this out, there would be a tech imbalance - i.e. it would take a few turns for the Vandals to find a city and overpower it. In the meantime, the other civs will have researched a couple of techs on top of their freebies. However, what I could do is make their starting techs Bronze Working (which is normally researchable after Tool Working, Mining and Warrior Code have been researched - but the Vandals would still have to research the other three techs to reap their individual benefits) and Boat Building, which normally requires a few other techs. That would give them a basic ship (in case they started the game on an island) and the spearman buildable after Bronze Working, which has 3 power against the 2 power that the basic Warrior has.