I want to know if I can become a vampire through Serena or her father and then at a later time cure vampirism and then go through the Dawnguard quest line? I am a bit of a hoarder and want to collect all of the items offered in both quest lines. Or will I miss out on certain items or quests?
The only way to complete both sides is to reload and play the DLC differently.
If i remember correctly i did side with the vampires initially going through the quest-line, however you get multiple options to switch sides in between (right up until the end). I switched sides after becoming a vampire and played as dawnguard and then switched again. In a single play-through you might get a bit of both sides but you'll definitely miss out on things.
P.S getting a cure for vampirism is a separate quest independent from dawnguard.