2) There are different mobs at different times, and vampires are around even during the day but there are a lot more at night. The vampires and devil bats I found give the hearts and the rest the brains. There may be more that give the hearts but that's what I noticed so far. You need 200 vampire hearts to 'end' the quest and any amount of brains though you need at least 13 to trade in for anything.
3) You will receive 120,000 adena for 200 vampire hearts and the option to trade in the brains for materials.