I've had mixed experiences at Viand and I've eaten there for lunch, for holidays and for dinner. I had my Thanksgiving there two years in a row and it was a fine meal. The last year wasn't as good as the year before but I still enjoyed myself. The thing that's probably going for you is that they do have a semi-private space that will probably work for you - it's off the right of the main floor on another level and is closed off by drapes, if I recall correctly. I've never done an event there before but I can tell you that the service I've had has pretty much always been good, certainly as good as Bistro 110.
You might consider Rosebud on Rush -- not sure about their private space options but it's a pretty nice version of Rosebud. You might also consider Roy's. Their private space would definitely accommodate your party and they have an interesting per person menu -- i think you could do $50 a person.
There is also C-House -- in the Affinia hotel? They also do not have a private space unless you buy out the restaurant (but could accommodate you in the back of house) but the room is really beautiful. They also have a bar/outdoor space on one of their higher floors that has an incredible view and would probably be perfect for 20. Might be a really special thing for you. I thought the food there was really very nice.
Roy's
720 North State Street
Chicago, IL 60654-3820
(312) 787-7599
C-House
166 East Superior Street
Chicago, IL 60611
(312) 523-0923
Rosebud on Rush
720 North Rush Street
Chicago, IL 60611-2504
(312) 266-6444