First I found a tool that could be downloaded from VMware (ovftool), I had installed it and used addtionally some modification - but I could not convert the VM.
ovftool /myPath/my_vm.vmx /myNewPath/my_new_vm.ovf
After visiting of some forums, I decided to install the trial version of VMware Fusion, and with the command below I could convert at least my VM.
/Applications/VMware Fusion.app/Contents/Library/VMware OVF Tool/ovftool /myPath/my.vmx /myNewPath/my.ovf
That went without any problems (about 40 minutes to convert the 80 GB), then, I wanted to import the new my_new_vm.ovf file directly into the Virtualbox machine, unfortunately, came close to completion an error to the fore (SCSI controller not found). But even this, there is a solution. Maybe there is a shorter way, but I had to import the VM into VMware Fusion and then modify the hard disk controller from SCSI controller to IDE controller.
After this, I used again the ovftool to create a new VM that could be imported into Virtualbox.
/Applications/VMware Fusion.app/Contents/Library/VMware OVF Tool/ovftool /myPath/new.vmx /myNewPath/new.ovf
See my long path to success, unfortunately, the size of the system has given me problems, so that I had to outsource files to an external USB disk.
I found a very good description on the internet on the oracle blog that describes the whole process in pictures.