I personally think Vader probably does better than Sidious did. ROTJ Vader > Rebels Vader > ROTS Vader (suit) ~ ROTS Palpatine I believe its also stated that Prime Vader is a superior duelist to ROTS Anakin.
Ontop of this, he has scaling and feats to back it up, such as overpowering ROTS Sidious with the Force immediately after ROTS, and blocked Sidious’s Lightsaber, both well before his Prime. That means hes likely gaging himself at about ROTS Palpatine level if not higher. Even if ROTJ Luke is around Dooku-to-ROTS Anakin Level (And thats pushing it, I dont even have him at ROTS Kenobi level), he would need to be pulling most of the weight since we know that Sidious appears like a Blur to Anakin and Sidious force-choked Dooku from half the galaxy away. Hes confident that He+Luke can clear ROTJ Palpatine.
