I don't see the conflict between Lament of Innocence and the older Castlevania games... besides Legends, of course. The future dark lord is born as Mathias Cronqvist, and sometime after becoming a vampire, he takes the name Vlad Tepes. There is no "real" Vlad to displace. In Castlevania's world, Leon's former buddy is the one and only person to use that name.
The life story of 'vania Vlad doesn't match up with historic Vlad, but even before Lament, it was never intended to. The older manuals and games never said he was Vlad III son of Vlad II, Voivode of Wallachia. I think it was the manual for Dracula Densetsu that was the first to take a crack at giving him an origin, and it says he was a fanatic sorcerer who built a castle, summoned demons, and wanted eternal life by becoming a demonic king. Then the manual for Akumajo Densetsusays Count Vlad Tepes used taboo arts to summon an evil deity to give him power. Lament rearranges and elaborates on these events - he summons Death first before becoming known as Vlad or obtaining his castle, gaining power isn't as simple as Death snapping his fingers, and he's an alchemist instead of (or in addition to) being a sorcerer - but I think the fundamentals are remarkably faithful.