Have you tried making and drinking the Ultimate Vampire Cure to make your Sim human, then asking another vampire to turn your Sim back into a vampire and waiting while your Sim goes through the turning process? It sounds to me like you are relying on cheats, which isn't the same as using normal game mechanics and might not have the same results.
ETA: Sorry, it doesn't look like that will make a difference. I tried with Vlad drinking the cure and then turning back to vampire. It did remove all his powers and weaknesses but he had his original rank when he finished the transformation process, along with the power and weakness points from his original rank. Looks like there's no hope for your Sims who already have too many weakness points.
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