You should try and feed on NPC´s they are delicious(and the new instant kill animation is glorious), it increases your stage, BUT a WARNING: be sure you have the skill mesmerize unlocked, at stage 4 you must use that skill on vendors or most of them will suspect you are a monster and will NOT TRADE with you, using that skill on the npc hypnotizes them and you can trade with them, you must look at them whileusing that skill, it gives an "aura" around their head (hard to explain). Some vendors will trade with you i think cryodil (not sure) and i think where you can vendor stolen stuff (not sure, i dont go to stage 4 at the moment, missing said skill point).

If all else fails, try the bloody maras? Those are not so expensive and can decrease / increase your vamp stage.
If they don't work, fill a ticket, it's a bug.