[quote=@Expendable]
So is there a source to this magic?

-snip-

So like vampires have a lot of mana after feeding?
[/quote]

I do have a tentative write-up [url=https://www.roleplayerguild.com/posts/5423022]here[/url], but to answer the specifics here:

The source is vitas; it's internal and can thought of as an 'innate' life. You're born with vitas and a vessel to hold it. As you age, the vessel breaks down until it is incapable of holding anything and you croak.

Using magic is to intentionally spill out some of it. It doesn't come back on its own. Instead, it's gotten by killing (or being in the proximity of) supernatural entities. As long as you're alive, you can cast but you can also definitely feel death coming. You could technically live sacrifice it, but poor efficiency, you'd be blacklisted from every morally good actor, you'd require a specific understanding (see: magic) on how to break a vessel, etc. Basically, the only way that would reasonably come up is killing supernatural.

Immortals have near infinite vitas (hence why they're immortal) so their magic is limited to what their vessel allows them to output (it can't be stressed, unlike a human) and their logic (an immortal still can't fathom being able to instantly wipe out portions of the city--most of the time). A vampire immortal would therefore not really have a change in it when they suck.

Vampire (as in the supernatural entity) are like all supernatural entities; they're beings [i]of[/i] vitas and not vessels that contain it. They just consume it so they're pretty much at max strength 100% of the time (for better or worse). They'd still follow the characteristics that people view of vampires, but it has more to do with their internal programming (how they are viewed).