The Booleans appear to be a bit buggy. I've found that deleting ANY element from a boolean difference will lock Vue up bigtime. I've never tried swapping the order, but it probably has something to do with the same bug. The solution is to UNgroup the boolean first, arrange the elements in the right order, then apply the boolean again (remember that it's always the first element MINUS the second etc.) There's another (unrelated) bug that will cause the same kind of lockup: NEVER try splitting materials on a mesh object that is a child in a group, move it to the ROOT of the layer first or Vue will have a seizure!