I noticed this on an earlier playthrough and thought I probably screwed something up, but it's doing it again where I haven't really changed anything (I doubt changing a history caused this - I haven't touched his dialog):
If you intimidate vandal after you free Lily, he gives you a free pack of blood.
Now, if you talk to him again (Edit: back at his window), you'll get: 1) buy blood 2) leave 3) Where do you get your blood from?
If you choose 3, you can intimidate him again, and get another free blueblood pack. This keeps going until one of his triggered conversations. But if you choose 'hope I don't kill you later' and leave without ever opening the store, the 'triggered conversation' never happens, and you can keep getting free packs like he's a broken ATM.