The reason you are getting confused is that you are working on the 6th edition rules. Check the full 7th edition Rulebook. Nowhere does it state that you cannot wear armour and cast magic, so there is no explicit rule stating that you can cast magic whilst wearing armour.
The only rule that you have to pay attention to now, it that you can only take armour if you have access to the mundane equivalent. Vampires do have access through their bloodline powers, so they are all fine to use mundane and magical armour and go on a casting frenzy.