DO NOT UPGRADE to MacOS High Sierra.  I emailed MOTU since Vienna hasn't mentioned anything and this was their reply: 

I'm sure MOTU has already received a lot of request for help on Vienna Ensemble Pro 6 MAS not working in DP 9.51 for MacOS High Sierra and there hasn't been any word from Vienna either about resolving this issue that has been going on for a month now. My question is what time frame can we expect for there to be a new update to fix this issue?

Added by Travis on 2017/11/30 16:59:47 Universal HI Lee, 

Thanks for writing. We appreciate your patience with this. At this time, VSL and MOTU are working together to get this issue resolved. I'm not sure when an update will be available, but I expect it to be before too long. 

I'll be happy to send you any information I have as it becomes available. 
Best regards, 

Travis Hagan