I am with the dawngaurd and I just did the bit in soul cairn. If I do part ways with Serana where is she so I can pick here back up later?

Yes and to be honest, I always part ways with her. I think the game's bugged as I do a lot less damage in VL form when she's around.

where is she to pick up again?

If you're in the Soul Cairn, I don't think you can part ways. It's a scripted instance where she needs to be there, at least for the first time.

outside in skyrim where can i pcik her up again if a parted with her

anyone?

Most likely Dawnguard Keep

Have checked Fort Dawnguard? You know, where you got her in the first place?

ok se was at the quest never mind

I parted ways with her also, waited 24 hours, and she wasn't at the Fort OR the Study or anywhere. After doing the next quest alone she showed up when I fast traveled back to the fort. The strange thing was she was following me as if I had never dismissed her...