My personal interpretation was always #1. I remember David Hill once mentioning something along the lines of vampire souls being more tied into their physicality, which supposedly is what allows regular Diablerie in the first place and Thousand Years of Night supports this somewhat by saying that diablerising a mortal is possible but noticeably harder. This to me hints that a vampiric body without a soul cannot exist at all. When a Kindred loses their soul (or what passes for it) he instantly turns to ash....