A data layer is really based on channels, so to map to the correct model you will need to setup Vixen to mirror xlights, or have xlights mirror Vixen. So, if a mini tree is channel 1-100 in Vixen, it will need to be the same in xlights. When I used Vixen, I created one controller in the tree and just created a series of channels. All of my models in Vixen were setup with the same channels as in xlights. So, when I exported Vixen and added as a data layer, it just works. I hope I explained that correctly. I don't have Vixen installed anymore, but I think you can export only partial channels.
James