So, I have vanilla Skyrim (no mods, no expansions)
I noticed that I'm able to use my weapons while on my horse, which was a feature introduced in Dawnguard. I thought to myself, "That's cool that they patched that feature into vanilla Skyrim".
I was playing a few minutes ago, and outside of Riften I was approached by Durak who spoke with me about joining the Dawnguard. It gave me miscellanious objective to go seek the Dawnguard, but I haven't gone yet. I'll see if it actually lets me do the quest line, but apparently that's what it's letting me do here. Anyone had a similar experience? Or can explain what's going on in my game?