Phone [Q] having problems with my phone and rooting.

I have a lenovo p780 after the official udate (KitKat 4.4.2) I used vroot to root my phone. My phone got rooted but after a few days the root was gone.I tried to root through vroot again but it didn't worked.the vroot app has become my system app. So I cannot uninstall it. Therefore I tried many other ways to root but nothing worked. So i tried factory resest. But the vroot app is still there. Can anyone tell me what should I do to root my phone?