I'm currently working with Vixen software for Christmas lighting.
I'm not very good at writing my own code yet bet getting better at reading it every day.
I found this post by zparticle http://forum.arduino.cc/index.php/topic,135501.0.html
who has written a sketch for Arduino using the mega2560 and Vixen "I believe" 2.0.1 version of Vixen2.
I copied his sketch and changed one thing I was having trouble with in Vixen3. I changed the footer: VIXEN_END to just END and the buffer size in the sketch.
If you follow the link above, zparticle has done a good job in a series of youtube video's describing how to set it up.
I had mine working great and something has changed and I'm not sure what. I'm getting allot of random noise on my com port causing the lights to fire randomly.
Good luck with your project,.