Problem: When I open a model, it looks bad. I see holes or flickering where parts overlap.
Explanation: If the model you imported does not include BREP information, Vertex uses the tessellation provided by your CAD system. This tessellation might be done poorly, or with low-quality parameters. Try exporting your model in a format that includes BREP information or re-tessellating your model with a higher polygon count, if that is supported by your CAD software.
Another possibility is that Vertex may have had a problem attempting to tessellate from your BREP, and substituted one or more parts with the given tessellation: in this case, what you are seeing is part or all of your original model polygons.
If you see flickering, holes, or mixed colors at locations where two or more parts touch, this is a normal artifact in 3D rendering and can be expected in any CAD or visualization software. Higher quality tessellation can improve this by clarifying where parts are touching or are not touching, but the nature of 3D polygons means that they may compete with each other for visualization. You can turn off one of the touching parts if you need to see the other part better.