Vampirism in Erebus isn't a disease like in a more traditional modern vampire mythos, it is the result of a truly dark ritual that vampires perform fairly regularly. They don't subsist on blood, they devour the soul of their victim. Every use of the ritual is a particularly vile form of murder, that deprives the victim of both life and their afterlife. The ritual extends the vampire's life by nearly however long the victim could have lived, and also enhances physical and mental prowess, but it also dehumanizes him and makes him ever more monstrous as he delights in the destruction of others. For anyone to use this shows that their conscious is very lacking, and they need some greater reminder than a normal man in order to reveal to them their guilt and hopefully drive them to repentance. While it has never been said in this way, I tend to think that Lugus is not naturally vindictive and so his curse is probably an attempt to reveal to them the guilt that they hide form themselves in order to give them a hope of redemption, whether by drawing out their actual guilt or using physical pain to replace the emotional torment that such a depraved mind no longer experiences.
Lugus is extremely unlikely to give in to any extortion. He is the embodiment of Truth, Honesty, Openness, and Revelation, so blackmailing him is impossible as he would want all the secrets to come out.
Lugus is much more forgiving that Junil, so I could see him removing the curse from a specific vampire whom was truly repentant. However, he would never remove the curse from someone who would continue to act wickedly. As such, a repentant vampire would not be a vampire anymore, and would have to give up all the advantages his dark gift had given him. If an elder vampire who has lived far longer than his mortal life repented he probably could not live very long, but may be reconciled with the sun god in his death and possibly spend eternity serving him in his heaven.
While Lugus is much more likely to seek redemption and reconciliation that Junil is, he stops far short of Sirona. He seems to try to combine their aspects in fairly equal levels. Sirona would go out of her way actively seeking bring the wicked back into the fold and never seek to punish them. She would of course have them work to make life batter for others, but that would be expected of those who never sinned as well. If they sin again, she will forgive them again. It is extremely unlikely for Junil to ever forgive anyone, especially in life. Lugus would take a middle route, seeking to redeem them but being very careful about it. He would try to make the consequences of their sin very clear to them first, carefully testing those returning to make sure they are sincere and not trusting those who return enough to let them fall back again. He is not that likely to forgive those who keep returning to the same sins over and over. His emphasis is to reveal the seriousness of sin and enlighten us so that we would of our free will not choose to return to it. Lugus is still rather stern, but he is very fair and what harshness his has is directed at enlightening and edifying those to whom he may seem harsh. Junil would rather keep everyone in line, and is willing to sacrifice the few for the sake of setting an example to keep the many in the straight and narrow. Junil punishes for the sake of society, while Lugus punishes for the sake of the punished.
