Surface meshes such as *.tri files may be imported to your OpenVSP model under the Import… menu. In the case of meshes, both those created within OpenVSP and imported from elsewhere, if the OpenVSP model is saved with a MeshGeom present, the entire mesh file is appended to the VSP3 file which can quickly lead to excessively large file sizes. OpenVSP is also capable of importing and operating on open or non-watertight meshes.
When creating meshes outside of OpenVSP with the intent of importing into a model, you may sometimes find that surface normals are flipped or there are gaps in the mesh. In some cases, the mesh will fail to import altogether. In these cases, you should try to export the mesh in ASCII format which has been demonstrated to be a more reliable import format.