Possible solution for T48047.

It seems the idea of the Displace Modifier midlevel offset was not integrated in the vertex groups mode, as it was with the texture mode.

This might solve the issue (i.e. work like the texture mode), the question is if that is a behaviour people would expect. Maybe it would make sense to set midlevel default to 0.0f as most people might expect no displacement if the weight is zero.