AVS Video Editor, from which one of the vep file types originates uses .vep files to store projects, i.e. work in progress. The program itself supports export to several multimedia formats (including 3GP video), however you will need the original multimedia data loaded alongside the projects, because these contain just references.
A lone .vep file is more or alone is useless, because it does only contain settings and changes done, but not the actual multimedia content.
This means that your only hope to convert vep to 3gp would be to use the AVS Video Editor and have the original video data referenced by the .vep project.