lso, I'm not really sure, but if Harkon can make you a vampire lord simply by biting you on the neck, why couldn't he just do that to Serana? Or wouldn't that be considered "pure blood" or whatever they called it?
The Vampire Lord form is something that has apparently developed in Harkon's particular Vampiric "clan" over time - it's not directly related to the Coldharbour setup, if it was both Serana and Valerica would also be able to call on the Vampire Lord form when they felt the need.
I don't even know if Harkon would have had the Vampire Lord transformation during the period before Serana was sealed away, it might have developed in the time afterwards.
The lack of the powers provided by that form might explain why Valerica decided to opt for running from Harkon and hiding both herself and Serana away, rather than face him directly. In a direct confrontation he'd (supposedly) be the more powerful. If both women had the same abilities, they'd have him 2 to 1 (plus whatever minions they could muster).