Naomi was drawn to Vamp out of pity, as I see it, he was one of her sins. It was her research that robbed him of one of the only two things life guarantees (Birth and Death). He was unable to die, asking Raiden and Snake if they will be the one to finally kill him. He was suffering, he could feel the pain but could not die. Snake shot him in the fore head twice, Vamp felt that each time, every stab Raiden did, everything. Naomi injected herself with the same nano machines so she could cling onto life, keeping the cancer at bay, allowing her to live for longer, maybe she was scared?
I think Vamp was drawn to Naomi because he knew it was her research that made him that way and maybe it was only her research that could change him back to normal. The way I see it, that is why he referred to her as his Queen and didn't attack her, trusting her fully.
Before Vamp died, it seemed as he smiled, as Naomi gave him the thing he sought, after fighting countless battle and receiving more than a lifetime of pain, he seemed happy. This made Naomi take her own life as well. Maybe as sympathy for what she has done to Vamp?