The more I think about it, a Soulburning vampire could still remain immortal as long as they siphon the souls of others instead of their own, just as they do with blood.
Don't know what special magical powers this would give them, but its possible they could use the power of a human soul to protect them from their physical limitations and even break those limits.
Considering demons are technically extra dimensional to the physical world, even if they could reproduce it might not be in the same way as humans. But considering the MC can become a human-demon hybrid in the demon realm and retain that form in the physical realm, that indicates the two forms (human and demon) are compatible to some degree.
If that is the case then one has to wonder how this partial demon biology would influence the human biology and vice-versa.