Sidious pretty much just sent Anakin/Vader to do tasks using the darkside, without giving Vader even basic instruction in it. This in the end costed him dearly, at that point Vader was basically just a dark Jedi than a Sith. When Sidious finds out Vader was going to fight Kenobi, he laughs at Yoda saying Vader will crush him, likely because he expected Vader to fight as he would and I have little doubt that had Vader been given even basic instruction on how to properly use the darkside, he would had. But Vader just kept fighting Obi Wan as he normally would, just powered by the darkside and unbalanced.

Sidious himself in a comic tells Vader that you must master fear, anger and other darkside emotions to use them properly otherwise they will use you. In a newer comic Sidious challenges Vader to a fight and after quickly defeating him tells him he still fights like a Jedi and that a Sith will use his surroundings to defeat crush his enemy, unlike the Jedi which use just lightsabers because those require precision and just do the damage intended and not more. Indeed, it was fighting this way which allowed Sidious to defeat Yoda, had he sticked to the saber, he would had been slain by Yoda. His philosophy of sabers being more to mock the Jedi and the Force being the real weapon would had helped Vader a lot in his fight with Obi Wan.

Really, it seems just taking a few hours to give Vader basic Sith fighting instruction would had made all the difference. Obi Wan was able to capitalize on hours upon hours of training, and beating, Anakin in lightsaber sparring, specially when the darker impulses of Anakin got the better of him.

Had Vader adapted to weaponizing all his surroundings, using the darkside as his main weapon and treating the saber as secondary, I think Obi Wan would had been crushed. It would had been completly alien to Obi Wan compared to his training sessions and had Vader just kept using The Force against him through their duel, by sheer potential and darkside power, Kenobi would had been eventually overwhelmed