Which version of Vixen are you using?
If a pixel tree is the only thing you're adding, I would build the model in Nutcracker with a start channel of 1. Build your effect and export as a routine (VIR) file. (As of right now, if you build a sequence in Nutcracker, you export all channels into a VIR file.) Then in Vixen you can right click on the channel and time you want the effect, load routine, paste.
This will only work in Vixen 2.x.x or Vixen+. Vixen 3 has Nutcracker built into it, but I'm not 100% on how to use it.