Originally Posted by
xliioper
It will price them on a fare if the connection is less than 24 hours. You just have to use VRN-BOS-MXP in multi-city search and it will work. You can't use VRN-AMS-BOS-MXP in multi-city because DL website won't merge multiple segments onto single fare when using multi-city. It will try to fare each hop separately which isn't possible for this itin and thus the search errors out.

I've never seen delta.com price out an overnight connection of this length westbound. Normally it'll just fail to return a result.