The "identity" only makes sense if one of the factors is a scalar, and it's wrong even then. I think you want this, where f is a scalar function and V is a vector function:
[tex]\nabla \cdot f\vec V = \nabla f \cdot \vec V + f (\nabla \cdot \vec V)[/tex]
I finally found it, looks like I was missing another term in the right side v(Div(v)), altough this is 0 in my case because I am working with fluid dynamics. Now everything makes perfect sense, thanks anyway