If you think I've been playing D&D, Shadowrun and Rifts for a long time, that's nothing on the time I spent on Vampire the Masquerade and it's attendant World of Darkness Universe. Here's some of the debris.
The idea of a magic wielding Vampire is not a new one, I just came up with a mechanic for it that fits the game. When I got my hands on Dirty Secrets of the Black Hand I saw that another coven of mages had taken the Tremere leap without looking first. It didn't take me long to see the hole in this logic and come up with a workable solution.
The group of death mages who gave up power for unlife where Euthanatos. Skilled death mages, used to dealing with Spirit and Entropy, not fumbling around with Forces. Add to that they are firm believers in Reincarnation and what follows doesn't make sense. After the first one got embraced they would stop. After all in the Underworld a mage who masters Entropy can survive for centuries. So where's the benefit in being a Vampire? If you're going to be somewhere Paradox isn't a big concern and you're not in danger of dying right now, why take the plunge? There's nothing that Vampire Disciplines can do that a mage can't do as well or better with a Rote. And if you're going to be living in Enoch any ways, why make it harder on yourself by giving up your best tool to control Oblivion?
Three things to consider here: 1) True Magic is brought about by the Avatar of the Mage at the direction of the Will. 2) The Embrace, just like Gilgul, robs a mage of his Avatar. 3) An Adept of Spirit can force a Spirit into a Spirit Cage. What am I driving at you ask? Well in my campaign a few of those Nagaraja kept their Avatars in phylacteries and used rituals and talismans to stay connected with them. This allowed them to continue using their magic while also allowing them to develop their Disciplines. And the Lich doesn't even need to be in the presence of his Phylactery in order to use magic.
All is not sunshine and roses for a Lich though. Their bodies aren't able to hold Quintessence, they need Talismans to hold Tass and use it to fuel their magic. Use of True Magic is restricted by the availability of Tass. Also the Will of the mage is not as closely connected as it used to be. It requires a monumental effort of Will to use True Magic through a Phylactery. To cast a Sphere effect or Rote the Lich has to spend a point of Tass and one point of Willpower. Being separated from his Avatar doesn't allow the Vampire to conduct Seekings either. The degree of magical power he has when he becomes a Vampire is going to be the same throughout eternity. In addition to these limitations the Lich isn't able to handle Paradox as well as a living mage. Each point of Paradox gained causes one level of Aggravated damage that can't be soaked by Fortitude. A Paradox backlash is usually fatal to a Lich since this damage is added to any from the Backlash.
On the plus side a Lich isn't going to miss it's body all that much. Whenever it's destroyed the Vampire's Spirit goes into the Phylactery. From there the Vampire can create another body or use Mind or Spirit to inhabit another body. Of course if the Vampire suffers a Paradox Backlash in the process he will destroy the Phylactery. Most Liches prepare bodies that they keep close to their Phylactery to avoid just such a tragedy.
Think about it... Immortal, powerful and as hard to put down as a Mummy. In my campaign I have only ever used one. But even that was enough to put my players on edge for months.