A single Dalek warrior is an incredible threat. On its own a
Dalek is a violent but relatively unthreatening individual. The
Dalek is a devolved humanoid species that vaguely resembles a
mutant octopus. The Dalek fighting machine/Dalek combination is one
of the most terrifying and destructive things in the whole
The Dalek race evolved constantly until they became a threat not just to the lower species but to the higher races such as the Time Lords. Eventually the Daleks and Time Lords all but wiped each other out in a Time War.
Daleks are geniuses able to compute massive amounts of information very rapidly. They have a sucker attachment able to manipulate things at least as small as a computer keypad, a gun attachment able to exterminate all life forms and an emergency temporal shift allowing them to travel in time. This latter ability would have been more pronounced during the Time War and, in fact, they have developed the ability to power themselves from temporal energy.
Daleks can fly, self repair and have an extremely effective energy shield surrounding them. They are, in short, almost the perfect killing machines. It was estimated by the foremost expert on Daleks that a single Dalek would have been able to wipe out a significant proportion of the population of the mid 21st Century US single handedly. It is unclear if the Dalek's energy field would be able to repel a direct hit from a nuclear weapon but if it could or was able to evade such an attack there is no reason to believe that a single Dalek would not be capable of completely exterminating the entire human race.
Darth Vader is a Dark Lord of the Sith. Vader has telekinetic and telepathic abilities as well as extremely heightened agility, spacial awareness and precognitive abilities. Vader can kill using his telekinetic abilities over a distance of several miles (as has been seen by his execution of incompetent officiers on other vessels). Vader has an innate understanding of technology and can exert control over it. Vader also has much higher than average speed, strength, reactions and leap range. Vader also has the ability to fly or drive any vehicle far beyond the expected capabilities of the vehicle in question.
Vader is armed also with his lightsaber. The lightsaber is an energy based weapon that can cut through anything other than another lightsaber. There is some evidence to suggest that lightsabers can cut through energy fields.
In virtually every possible encounter the Dalek would win. Only The Doctor has proven to be an effective adversary to them.
Darth Vader would, despite all of the Dalek's abilities, win easily.
Vader's abilities would allow him to deflect the energy of a Dalek's attack using either his lightsaber or his inate abilities. He would be able to evade any other attack through precognition, agility and speed. He could cut through the armour of a Dalek easily with his lightsaber. His telekinetic abilities would allow him to move objects towards the Dalek or to move the Dalek itself. Vader could use his telekinetic abilities to take the Dalek's armour apart at a distance or he could take it over and force it to self destruct. He could also use the Force to simply kill the Dalek at a distance.
The only ability that Vader possesses that wouldn't help him to win would be his ability to influence the minds of others. The Dalek's singular will and phenomenal intelligence would be to great for him to exert any power over it. This would not be relevant in the grand scheme of things as he could kill the Dalek in so many other ways.