Serse Cosmi was the first dismissal on a fractious afternoon in Tuscany, the Bianconeri head coach seeing red after Ante Vukusic missed a penalty just moments before the break.
But Valiani's regulation finish on the half hour had already provisionally lifted the Robur out of 20th place for the first time this season, leaving Cosmi to fret over Sunday's Derby della Lanterna between Sampdoria and Genoa, who now prop up the table.
Meanwhile, an eighth defeat of the season leaves Pescara boss Giovanni Stroppa on very thin ice, with the dismissals of Marco Capuano and Damiano Zanon doing nothing to boost the Delfini's short-term prospects.
Vukusic introduced himself to Gianluca Pegolo after a couple of minutes, wriggling into the box to force a nervous save from the Siena keeper.
Alessandro Rosina was wasteful on the counter, putting the ball wide when one-on-one with Mattia Perin after Francesco Bolzoni's fine through-ball.
Pescara defender Antonio Bocchetti got across to cover Simone Vergassola's effort and Emanuele Calaio fluffed the rebound, but Siena would take the lead in the 31st minute.
Valiani kept things simple with a diagonal strike from the left wing and Perin was left standing.
Pescara were handed a rare opportunity to level in the 44th minute when Matteo Contini cut Vukusic down in the penalty area.
Pegolo stood firm to parry the Croatian's spot-kick, though, and Siena would retain their lead going into the break.
Cosmi would not return to the touchline after the break, though, an altercation with the officials seeing him consigned to the stands for the remainder of the match.
Perin denied Rosina with a spectacular save as play resumed, with Juan Quintero beating the Siena back four to test Pegolo in the 54th minute.
Both keepers were in fine form and Perin looked lively to repel Calaio before Quintero gave Pegolo another fright with a turn and shot that rattled a post.
With their keeper resolute, Pescara's outfield players started to lose concentration and Valiani was permitted some fine forays through the centre of the park.
The Delfini did manage to create several half-chances late on, with Elvis Abbruscato reasonably lively, but their chances of salvaging a point dissipated when Capuano picked up a second yellow card.
And Stroppa was made to look somewhat foolish for frantically re-arranging his backline as a second defender, Zanon, headed for an early bath for a high tackle two minutes into stoppage time.