I get the distinct impression that Vader was really a disappointment, and that he was more or less a “placeholder Sith”, a tool to be used, but not really worthy of becoming the next Sith lord. Sort of like Darth Maul, really, who was an attack dog, but was ultimately easily replaced by Dooku.
In fact, I gather that Vader wasn’t really even as desirable as Dooku (Darth Tyranus) as an apprentice, since his Force potential was severely curtailed by his injuries at the hands of Obi Wan. Also, Dooku was vastly rich, influential, and charismatic. Much more Sidious’s speed.
So, did Sidious only rescue Anakin from the lava and promote him to Sith status because “well, I don’t really have another candidate in the wings, and I can’t afford to lose ANOTHER apprentice. Shit, why did I have this schmuck kill Tyranus? Might as well put him back together until something better comes along.”