Indeed, Vatsim has nothing to do with it. (altough Dave may like it, I dont. Flew it in the past when it was SATCO, but the PC's werent strong enough to enjoy it. And I hate seeing deserted airports like I am in the Langoliers movie, and then listen to 14 year old kids playing ATC)
The ATC.DLL needs FS9.1 update, and helps a lot. Together with AI Smooth it will solve your problems. ALSO, oversaturation of the radios due to way to many AI of course will always be problem number one.