How to i convert a VirtualBox’s .vdi files to VMWare vmdk/ovf formt

Why not export the VM's as virtual appliances on the source host and import them on the destination host? That's the way I move VM's around and it works like a charm. (Of course I'm a complete VMware neophyte).

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