If you feed an Object's Vray Displacement Mod map into the Z-Offset in Forest Pack it will align a forest pack to the displaced terrain. However if you do this, you can't use ZOffset, you can't use slope and you can't use height limiting. You also have to calculate as a % of the height of a tree the displacement percent which is annoying when changing tree models.
It would be nice if you could have it look at the Vray Displacement mod and just assume that it's on the Z-Axis at least as well as generate a normal map internally for your slope calculations. When I've got 16k displacement maps it isn't really feasible to tessellate out the displaced terrain for ForestPack to analyze. And you're very close with the Z-Offset, you just need to add an additional Z-Offset with some basic math to scale it earlier in the order of operations.