Viviani's Lotto teammates tried to lead him out in the last kilometre of the 163km flat ride from Monzon to Zaragoza, but Philipsen, who had also won the first stage, pushed in the final moments to be the first at the finish line, even after losing contact with his own lead-out rider.
"I had to find my way, take a bit of extra wind, and really come late because my legs were concrete. But I just managed to win, so I'm really happy, and the effort of the team has not been for nothing," the 27-year-old said.
Joan Bou, Sergio Samitier and Jose Luis Faura established a four-minute lead on the chasing pack in the first 20km of stage eight.
Dylan van Baarle (Visma-Lease a Bike) tried to position himself in front of the peloton with 5km to go, but by the last kilometre, Lotto riders had the upper hand before Philipsen's final move.
Norwegian Traeen (Bahrain Victorious) remains two minutes and 33 seconds ahead of Jonas Vingegaard (Visma-Lease a Bike) in the general classification, with 13 stages still to go.
Philipsen's 15th Grand Tour stage win closed the gap on points classification leader Mads Pedersen (Lidl-Trek) to 12 points.