Hi David,
depending on your audio system (which you didn't mention), a good setting to start with should be 1024 samples for the audio driver and 0 samples for MIR Pro. For really big arrangements, it might be necassary to change the latter to twice the length of the system's latency. For smaller arrangements (i.e. less MIR Icons on a Venue's stage) it might be possible to use lower values.
In VE Pro's Preferences, assign as many threads to its engine as possible. The best setting depends a lot on the actual context: As long as you do most of your mixing and processing in VE Pro, this number can be high. You will have to reduce it when you plan for more processing in Logic.
HTH,
/Dietz - Vienna Symphonic Library