Dooku was more of a duelist than a Force practitioner. Vader seemed not to be particular toward either one.
Dooku has: held his own against Yoda (he lost in every encounter he had with Yoda though even while amped), sparred as an equal with and beaten Mace Windu,beaten Obi-Wan and Anakin consecutively, beaten Tholme and Sora Bulq simultaneously, beaten Quinlan Vos,beaten Grievous several times and beaten Asajj Ventress
Vader has: beaten the Dark Woma, beaten Celeste Morne, beaten Roan Shryne, fought as an equal with Darth Maul, fought as an equal with Luke , fought as an equal with Obi-Wan (whose skills had increased since Mustafar), effortlessly beaten five or six Jedi, two to three at a time in succession, beaten four out of eight Jedi; one of them killed another; the last three defeated Vader by combining their powers to hurl objects at him (this was shortly after RotS; so Vader was still new to his armor and substantially grew in power and skill later on), beaten countless other individual no-name Jedi and Dark Jedi (Bol Chatak, Sha Koon, Sardoth, Hylon)
The opponents Dooku has fought have their own respective feats to credit their skills, while quite a few of the opponents Vader has dueled are virtually featless, which is largely why I consider Dooku a slightly better swordsman.Vader's Djem So could possibly even that out.
That said, Vader is more powerful and versatile than Dooku, but Dooku has impressed me more as a duelist.