Both .vbl and .gdf files are used for 3D geological models in 3DMode software. As such, it is likely that they can be converted between each other or perhaps to some generic exchange format. Alas, we couldn't find anything specific regarding this conversion.
Convert 3DMove 3D geological model file to 3DMove 3D geological model file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert vbl to gdf on Windows
A structural restoration, validation and analysis software
Convert from vbl file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.