Obviously i've already joined the Dawnguard, as that is the first quest. So now I've led Serana back to the Castle, and her father is asking me whether I want to be a vampire or werewolf...since I've already joined the Dawnguard does becoming a vampire in essence end that storyline, and i take over the vampire storyline?

Also, if i choose Dawnguard, do I have later access to Soul Cairn somehow, because I want to see that place. Thanks.

Both Vampires and Dawnguard head to the Soul Cairn, and yes, choosing to accept Harkons Gift will result in you starting the vampire storyline and end the Dawnguard storyline.

Thanks

Won't spoil it for you but yes choosing the Dawnguard you still get access to the Soul Cairn.

You won't be sorry if you stay in the Dawnguard. Trust me.

Try the VL on another character, or just save before finishing the dialogue (well, by now it's probably too late).