theres some foreshadowning on it. the original hellsing met dracula in shambles, grieving the lost of his loved one. for some odd reason he decided to keep him in the hellsing HQ and call him their ultimate weapon.
truth be told, it really looked like both knew that integra was gonna be his lover reincarnated, and thus alucard agreeded to "wait" until then. alucard is really dracula in any way, wich just raises the mary sue levels off the chart.