Anakin/Vader has never surpassed Yoda's skill in lightsaber combat.
In Attack of the Clones, Anakin lost his arm to Dooku. Dooku would have lost to Yoda if the former did not distract the latter in order to escape. Therefore, Yoda > Dooku > Anakin.
In Revenge of the Sith, Anakin lost to Obi-Wan. Since Yoda is considered one of if not the best swordsman in the Order, I think it is safe to assume that Yoda > Obi-Wan > Anakin.
Anakin went through the following changes in his transition to Vader-hood:
- His potential in the Force is diminished. Palpatine once told Yoda that Anakin "will become more powerful than either of us". After Mustafar, though, Palpatine concluded that Anakin will never reach his full potential.
- Vader's suit inhibited his prior combat style. The Jedi tend to favour more acrobatic styles, but Vader's suit has a lot of bulk and deadweight, slowing Vader down.
- To compensate, Vader changed his style, sacrificing the speed he already lost in favour of power and accuracy. In time, he adapted and honed on those aspects further, as well as improving his physical stamina.
It is still fairly easy for Jedi survivors to evade Vader's attacks, but Vader is still able to keep up enough to press the attack and wear them down - he no longer creates openings like before, but simply waits for one. Having enough speed and agility to evade his attacks is one thing - having enough of those attributes to overcome his defences is another. It is, however, a weakness that Yoda's speed, agility, combat style and skill in the Force is sufficient to exploit. Therefore, if the faster and more agile Anakin couldn't win before, he most certainly couldn't ever do so as Vader now, even in spite of the changes and improvements with age.
You may argue Yoda is older during the Empire, but firstly, age doesn't affect a Jedi much, given the restorative properties of the natural Force. Secondly, what is another 20 years to a 900 year old? Yoda only showed signs of age and impending death after Luke arrived on Dagobah (and probably introduced a foreign bacteria that Yoda wasn't vaccinated against).
The Wookiepedia entries for Anakin Skywalker and Yoda describe their combat styles. If you accept Legends information, the Legends sections go into more detail. The Legends section on Darth Vader's suit also goes into more detail how it affected his style in combat. The Legends history of lightsaber combat is an overview that highlighted their styles as well.