You can use any converting tool listed in our conversion list, like SUPER, Xilisoft DVD Creator, Adapter for Mac etc. For various platforms is also available free FFmpeg encoder. To convert 3G2 to VOB with it, open terminal (bash, command line) and type:
ffmpeg -i /path/to/your/file.3g2 -vcodec copy -acodec copy /path/to/your/newfile.vob