Space between widgets in QVBoxLayout

The issue is not the margin settings of the QVboxLayout, but the margin settings of the layout of your container widgets and the spacing setting of the QVBoxLayout. You already have set the spacing to 0, this should be fine. In addition, assumed that upperWidgetLayout is the layout of the upper widget and lowerWidgetLayout the layout of the lower widget, try

upperWidgetLayout->setContentsMargins(-1, -1, -1, 0);
lowerWidgetLayout->setContentsMargin(-1, 0, -1, -1);

This sets the bottom margin of the upper widget's layout and the top margin of the lower widget's layout to 0, so that there is no space between the contents of the two widgets:

Temporarily coloring the various widgets is usually a good approach to track down such issues. You can also use Qt Designer to design the UI and have a look at the source code which is being generated (or use the .ui file directly in your project).