Alrighty so I've started a new character to experience Dawnguard, I haven't played through any of it so far.
I've progressed through the main story until the elder scrolls part where my character decides to become more experienced before venturing forward.
I haven't joined the companions and I was working on random quests in Whiterun and while on my way to north watch keep to free Thorold, I was attacked by a wave of bandits, then a wave of vampires!
Long story short I have Sanguinare Vampiris.
I haven't been approached by a courier, only overheard about the Dawnguard from others.
So my two questions are, if I retain my vampirism:
1.) Can I still remain a vampire in secret and become part of the Dawnguard?
In other words could I still start the expansion/path to vampire lord?
Also when joining the companions as a vampire what would happen during the ritual of the circle?
I'd really appreciate knowing how to move forward!
Thanks guys!

You'd eventually have to be cured if you want to join the dawnguard.

For the companions, you would be turned into a werewolf, but you can always cure yourself of it later.

Thanks!