Vampires need to get blood somewhere and maybe because of that they have to form hierarchies and organizations, like in the World of Darkness setting. A Lich can just sleep for millennia if something is wrong - as Vordakai did. Surely, it has its own drawbacks, but still.
Think a bit on how is to (un)life as a lich. No ability to fell pleasure from touch, food, enjoy a wine, deformity forcing you to use illusion magic every time that you go into a city and if someone steals your philactery, you are his slave. Now, a high level vampire wizard focused on necromancy, he probably can easily become noble, owns a lot of servants and as he is a high level wizard, sun is not a problem(just control weather). I'm not comparing the average lich with average vampire. I'm comparing the (un) life which a lv 17+ caster would have becoming either of the two undeads.
You need to create a story relevant to the biography of the main hero (including all the secrets). It has to be believable and cool. You need to write separate plots and dialogues for all the companions and some NPC's as well. And I am sure you need some (or maybe a lot) of graphical assets to add to this Path. Probably even some completely new NPC's with their own dialogues, portraits and models.
I know. If I do it, would be a later mythic path, something simple, like if you refuse Zacharius ritual, he become hostile, you kill him and in his corpse you find a "ancient vampire blood" sample and research notes(he said that he researched vampirism too, it would make sense) and find a story about how the first vampire was a mortal who resisted the corruption of a succubus. Drinking that blood would make you a vampire.
The vampire template already exists in the game. Graphical assets? Just replace teh skeleton pet with a shadow demon pet. Give undead and vampire template. +3 on illusion DC, weakness vs holy and fire, makes illusions more real and the path is implemented.
The problem is that I don't know how to do it. I'm a code monkey IRL but don't have much experience with modding.
Now, if the vapmire mythic path is since chapter 2, then yes, would be insanely hard to implement it.