How can I convert a VMDK to OVF?
The VMDK file is the actual virtual disk, an ovf file is just a settings file (plain text), that VMWare products (like ESXi Server) and other virtualization software (like Virtualbox) can use when importing the VM.
You may have an existing settings file (like a .vmx) which you can convert into ovf format using VMware's tool 'ovftool': http://communities.vmware.com/community/vmtn/vsphere/automationtools/ovf
There's quite a lot of options, which are detailed here in the user guide: http://www.vmware.com/support/developer/ovf/ovf20/ovftool_201_userguide.pdf
You can use VMware's Workstation Pro or thier OVF tool. In Workstation, load the VM then under file menu, select export to OVF. Be aware this can take a long time. VMware's OVF Tools is a command line option.