In a sequel novel to Dawn of Sorrow, Olrox was the antagonist. It’s not canon, but considering the sorrow series makes references to Olrox (in clothing items), it wouldn’t be surprising if he was still around.
Just a little comment about Olrox because this comment reminded me of some thinking I've done on him.
I think that the biggest reason that Olrox is widely pegged as being a sort of unofficial heir to Dracula's throne is because out of all the vampires we've seen in the series who are not Dracula, Olrox's abilities --both in nature and in scope-- veer closest to Dracula's. He's very much a Dracula-lite. Furthermore, once upon a time he was a close enough ally of Dracula to be given partial command over Dracula's forces and maintained his own part of the Castle.
Also he just looks cool.
Wouldn't surprise me if he was still around, but who knows, maybe Alucard killed him for good in Symphony.
Also, just because there's no more Dark Lord doesn't mean that the Darkness stopped coming -- it's just no longer centralized under a Biggest Bad. There will always be monsters as long as mankind has darkness inside them.
Vampires will continue to rise.
Rest assured, the Belmont family will forever be gainfully employed.