Pieters (Wiggle High5) sprinted to victory ahead of Lisa Brennauer (Canyon-SRAM) and Vos (Rabobank-Liv) in a photo finish in the centre of Stratford-upon-Avon at the end of the 140km stage from Atherstone.
Orica AIS rider Gracie Elvin was fourth and overnight leader Christine Majerus finished in fifth place.
"It was a hard day, a lot of rain and some quite steep hills. We did a really good job with the team, we have such a strong team here this week so we can play a lot of cards and I'm really glad that I could finish it today,” Pieters said.
Vos now holds a three-second lead over Majerus, with Pieters a further four seconds back.
Despite often inclement weather conditions the stage was another fast paced affair, with lots of attacking riding, no rider more so than Ale Cipollini's Emilia Fahlin, who was twice away in moves off the front of the race, the second time alone.
The Swede was caught on the first climb, with Amalie Dideriksen (Boels Dolmans) and Malgorzata Jasinska (Alé Cipollini) the next duo to stay away, building a gap of 50 seconds, which was steadily whittled down by the speeding bunch.
The pair were caught by the small lead group with less than 20km of racing remaining and despite a flurry of attacks on the run in, the leaders hit the final kilometre together, with Pieters pipping Brennauer by the narrowest of margins.
Vos also moves into three point lead on the points classification over Majerus, who is ten-points clear of Pieters.
Lizzie Armitstead (Boels Dolmans) finished safely in the front group in 18th to remain the highest placed British rider.
The mountains classification also changed hands, moving to United Healthcare's Katie Hall, two points ahead of Ilona Hoeksma (Parkhotel Valkenburg).