I need to reload an old save so that I can get the vampire master trophy, but the only save I have is from just after I exited the soul cairn.

So my question is can serana turn you into a vampire lord after you exit the soul cairn?

Thanks

Yes, after you have completed the Dawnguard main quest.

Yes, from that moment on, the dialog-option should be there.

To be exact, from the moment of entering the soul-cairn.