I use MyTraffic 6 and it works great in P3Dv3. Just make sure to follow the installation instructions closely. Also make sure to keep the AI traffic slider really low (mine is at 5%).
Not sure how it works with VATSIM, though, since I presume nobody else (ie. the controllers) sees the AI traffic on YOUR system. Could be wrong. I'm guessing you can have it in the background but it won't be affected by you or your instructions.
MyTraffic 6. I recommend it.