I would have personally sought out a plastic barb for the chemical compatibility reasons you state (one less variable to worry about) but if it's glycol type coolant brass will be fine. Also there may very well be brass elsewhere in the system informing their decision.

Vibration is a non-issue. Pipe threads are is 200yr old tech. Pipe threads in all manner of materials have been in service in all manner of environments the world over. Some of them for a century or more.