I will install a new vanity lights (three lights) in my bathroom. The weight is over 7.5 lb. There is a box on the wall and I am not sure it has stud behine it or not. Is it too heavy?
IMO, 7.5 lbs isn't too heavy - provided the junction box is adequately secured to a wall stud. If not, is there a way to anchor the fixture other than only the connections to the box?
You might want to consider using a ceiling fan box or brace. One type can be blind-installed in a ceiling (or a wall) and will grab on to the studs on either side of the opening.
"You might want to consider using a ceiling fan box or brace."
Even Madison straps will easily hold a 7 pond load.
A fan brace would really be overkill (and might show if the lights are deigned for a device box mounting instead of an octagon box mounting).
My contractor bought a ceiling box but I guess he screwed it to drywall and no stud. That is what I am concerned about. I switched a lights which is lighter - 5+Ib. He will put there next week.
The drywall can easily support these small weights.
Sink a few screws from the fixture into the drywall and forget about this.
Is it a vanity fixture with 3 lights or 3 separate fixtures? Your description of "a new vanity lights (three lights)" has me wondering.
How many screws are holding it up? A couple of small nylon expansion screw anchors should hold it if there is any question.