So I got to the part where Lord Harkon asks me if I want to become a vampire or refuse his offer, during this my master thieves guild hood was still working. I accepted to become a vampire and then after that my hood gives me a bald spot and it's now invisible for some reason. I don't know why this is so. It happens with every mage hood and the normal thieves guild hood. The only hood that is working for me is the nightgale hood, I don't know about the dark brotherhood's hoods or any dragon priest masks yet, however I suspect the same or similar results happening. Can somebody please help me?

So this Dawnguard destroys other Skyrim content that you may love and adore and wear all time? Yey!
I can't help you and I bet only Bethesda can answer this but as they don't do customer support, I don't know.

Bug introduced by the 1.6 patch, not a Dawnguard issue.

Thieves Guild Hood
Guild Master's Hood
Linwe's Hood
Blades Shield
Ebony Shield

Those items have display issues on Vampires.

I wonder if the new bugs that get introduced in each patch that fix 1 or 2 quests, is longer then what they fix? It looks like it so..