You can use the cheats, but it'll disable the achievements...
Try increasing the Vampire Lore skill (Start by searching the information of vampires on the computer, then buy and read vampire tomes), so your vampire can prepare a plasma salad. Make a lot of them, maybe they'll increase vampire's weight. I'd recommend you to max this skill out, as some of the things your sim can do are potion (such as vampire cure), ability to buy and make plasma packs and of course quicker vampire XP gain.