The hosts took an early lead through a Jose Ze Castro own goal, but Alberto Bueno levelled things up with a goal against his former club before half-time.
The result means Valladolid will end the weekend still in the drop zone, three points worse off than their opponents and with just one win in their last eight matches.
Rayo came into the fixture on the back of three successive wins, but could have trailed after just 20 seconds on Saturday when Javi Guerra fired narrowly wide of the left post.
The opening goal followed after eight minutes, with Jeffren's corner from the right exposing weaknesses in the visiting defence which ended with Ze Castro slicing the ball into his own net.
The first half was drawing to a close when Rayo finally began to show signs of life, although Bueno could only direct his shot past the post.
However, the forward made amends on the half-time whistle, latching on to a cross from Roberto Trashorras before smashing a volley into the bottom right corner.
Trashorras whipped a free-kick just past the post as the second-half action got under way, while Oscar had Valladolid's next chance but volleyed wide after being played in by Valdet Rama.
On loan striker Ruben Rochina then seemed set to give Rayo the lead after 76 minutes, only for home goalkeeper Jaime to tip away his left-footed curler from the edge of the area.
It was a battle to the last as both teams pushed hard for the winning goal, but Rayo keeper Ruben was equal to Guerra's late effort while substitute Daniel Larsson fired wildly over the bar after being teed up well by his Valladolid team-mates.