After five successive wins, including a memorable Carling Cup victory over Preston North End, they were beaten by an early goal from Paul Terry, older brother of the England captain, John.
In the fifth minute Terry picked up the ball just inside his own half and weaved past two defenders before firing a magnificent 25-yard drive past Mark Goodlad.
Not many would have backed the midfielder to get the game's first goal, especially as the Valiants' front two of Leon Constantine and Akpo Sodje have been forming the most feared partnership in Division One.
Constantine, who earlier in his career played in the West Country for Torquay United, returned to his old haunt looking for his sixth goal in what promises to be a prolific season.
Although the home defenders soaked up a lot of pressure, there were opportunities at both ends with Chris Cohen a powerhouse in the Yeovil midfield, and Paul Harsley twice going close for Port Vale in the first half with efforts that skimmed the bar.
After the break Vale threw everything into wave after wave of attacks and Constantine could have grabbed an equaliser in the 66th minute but the ball flew off the side of his head and went for a throw in.
An even better chance fell to Sodje six minutes from the end. Substitute Joe Cardle provided a perfect cross from the left but Sodje planted his header into the ranks of Vale fans behind the goalmouth.
Yeovil also had two excellent opportunities to double their lead in the last six minutes. First Cohen got clear on the edge of the box and was about to pull the trigger when he was floored by skipper George Pilkington.
Then, deep in stoppage time, Yeovil sub Kevin James seemed certain to score when he found space on the right, but his shot flew wide.