Let me see if I've got it:

[list]
[*] The source of all magic is life force, known as "vitas";
[*] Vitas is in all living things;
[*] Vitas in humans degrade over time (we're leaking vitas);
[*] To use magic, we have to spend some of our vitas;
[*] Supernatural creatures crave vitas;
[*] Killing living creatures releases their vitas;
[*] Supernatural creatures kill other living creatures to get their vitas;
[*] Killing supernatural creatures releases their vitas to those around them as they die;
[*] Immortals are glutted with non-degrading vitas and attract supernaturals towards them;
[*] We're imprisoning Immortals in iron caskets to keep them from attracting supernaturals.
[/list]

But if supernaturals crave vitas and we, the human race, are leaking them, doesn't that make cities the next-best source next to Immortals?

What happens to the vitas that humans and other living creatures leak? Does it lie about (perhaps collecting in pools to create the fountain of youth, healing springs, etc.), or is it collected by something?

Does that lead to the birth of supernatural creatures? Or does that get sucked into immortals, so big cities act as charging stations for immortals?

If Immortals suck up all that leaking vitas and we lock up the immortals in metal caskets, does that uncollected vitas spawn another immortal?