Actually I believe it was stated he made a pact with an "evil deity," which was most likely Chaos, not the Christian Satan. That may have been a translation error, as in Aria you get "Satan's Ring" (description: "Satan himself once wore this ring"), and the series is no stranger to translation errors. But all this is up to interpretation, there may be no right or wrong answers.
I believe in the CV universe Lucifer never existed, it was always Chaos. Dracula chose to become one with Chaos, thus beginning the Dark Lord legacy.
Actually it doesnt state anywhere in the manual that he made a pact with the devil or an evil deity so we both stand corrected on that unless stated in the japanese manual which doesnt hold much ground with me being Im an american. I understand what you are saying, that evil deity=chaos but given the amount of "christian" references I would have to say if he did make a pact with an evil entity then one
at that time would have to assume it was the devil. Even Sonia stated "..until your soul is saved." Which is a common Christian saying and given they use Crosses,, holy water, and other traditional christian methods of slaying creatures of the night and Trevor himself and if im not mistaken even Reinhardt was found praying at the start of both of their games though i believe Reinhardt was more or less asking for strength.
EDIT: Plus at the time the whole Chaos and Dark Lord theory wasn't introduced since it was IGA that did that with Aria though hinted on in SotN. Until then Dracula was just an evil vampire pissed at the world.
But again there is no right or wrong answer being that legends was retconned and the actual evil of the world that exists in castlevania, at least until lords of shadow drops meaning IGA's timeline, is Chaos.