Vampires take radiant damage from direct sunlight. Not magical light or dancing lights. You need things like the 6th level Daylight spell or a Sunsword. And they regenerate when not in sunlight unless they've taken radiant damage in the previous round. That encounter should be a TPK for 4th/5th level adventurers if the rules are followed and the vampire is played intelligently. It wouldn't even need its spawn. That said, in my attempts at this battle the window coverings have been completely untargetable so maybe that could work against the vampire if they could be broken... if it's stupid enough to go near the windows and not just pick off characters that stray outside the small area of protection.
I know the game vampire is "only" CR 8 instead of CR 13 for the SRD version, but even so. The CR 5 Vampire Spawn in the SRD are only vulnerable to direct sunlight also.