The 22-year-old, who spent the second half of last season on loan with Real Zaragoza, has featured just three times for Rovers since he recovered from a dislocated shoulder in November.
As a result, he will link up Vallecano until the end of the current campaign, with the La Liga outfit having the option of making the switch a permanent one during the summer.
"Obviously we are seeing it as an opportunity for him him to go out to Spain and get games," Blackburn boss Gary Bowyer told the club's official website.
"His injury on the opening day of the season set him back with us. So we've agreed it will be good to get him playing some games and we'll pick it back up in the summer.
"Playing in his native Spain appeals to Ruben, so we'll monitor his progress."
Rochina joined Rovers from Barcelona B in 2011.