Los Che continue their superb record of scoring in each La Liga fixture as they put pressure on Real Madrid and Barcelona atop the table after strolling to victory at Vallecas
Jonas provided the only action of the first-half with a superb strike from the edge of the area and Tino Costa doubled the lead for the visitors after 57 minutes.
Rayo scored in the final 10 minutes through Raul Tamudo to ensure it was an uncomfortable end to the match for Unai Emery's men, who bounced back from last weekend's league defeat to Real Madrid.
Neither side were able to control the opening stages of the match, with possession constantly changing from one side to the other with few chances being created.
After 20 minutes where there hadn't been a meaningful strike on goal, the ball fell to Jonas who hit an unstoppable, low drive into the corner of the net to put Valencia ahead.
The match turned stagnant following Jonas' strike with a sustained period of play where both sides adopted an ugly style of play, hoisting the ball forward in the air.
Sandoval's side did manage to work their first shot on goal after 40 minutes when Jose Movilla's freekick was headed goal-wards only for Diego Alves to tip it around the post.
The Brazilian shot-stopper was on hand to save in spectacular fashion from Andrija Delibasic early in the second half after Piti's precise cross found the Montenegro striker, who managed to direct a header on target.
Valencia doubled their advantage through a beautifully crafted goal consisting of nine passes in a perfectly executed counter attack initiated by David Albelda.
The break ended in Roberto Soldado combining with Sofiane Feghouli in a neat one-two. The Frenchman then slid the ball to the far post where Costa was on hand to send a lethal shot into the opposite corner past a helpless David Cobeno.
With the match approaching full-time, Rayo Vallecano were given a lifeline five minutes from time. Substitute Raul Tamudo bustled his way into a scoring position before poking the ball beyond Alves to halve the deficit.
Valenica had opportunities to re-establish their two goal lead in the closing minutes but it was the home side who finished the stronger as they went in search of the equaliser, only to fail in their pursuit with Triguero Tito passing up a glorious chance to snatch a point at the death for Rayo.
The win moves Emery’s side to within four points of Real Madrid, who play later on Saturday evening.
Rayo remain in mid-table but travel to Camp Nou in their next game on Tuesday to face Barcelona.