The Rounded Rectangle cross-section has some additional behaviors that are worth noting. The Key Corner function of this cross-section will enforce the interpolated W lines nearest the intended corner to lock into that corner thus ensuring a sharp edge if desired. Adding Skew or Keystone will alter the true corner radius and if set such that sharp corners are enforced, the radius will automatically return to zero. Finally, users tend to use Rounded Rectangle for fuselage sections and have issues with VSPAERO VLM runs. The reason for this is that there are perfectly vertical sides in these sections that are collapsed to a single edge in the degenerate geometry which will crash the solver. For the purpose of VLM analyses, it is better practice to convert these sections to an ellipse so the cruciform plate is maintained without collapsing the sides. You may also try using a Superellipse with M and N set to 4.0 or a General Fuse section with all strengths set to approximately 1.5.