Uche almost supplied a goal for Moi Gomez, whose curling effort did not bend enough to sneak inside the far corner, before Vietto went close with both a header and a long-range shot.
Cassedesus almost gave the visitors a half-time lead, but his header from a corner was well saved.
Vietto broke the deadlock in the 56th minute when he blasted the ball home from close-range after Villarreal upped the pressure after the break.
Uche and Gomez both went close to making the points secure before Levante started to press forward in search of an equaliser.
However, the home side, who were without Arsenal target Gabriel Paulista in defence, remained solid and claimed victory.