vmware's ovftool can turn vmware virtual machines into OVF format, which
with some massaging can be used to generate a virtualbox virtual machine.

this package is available from the vmware website and is named something
like:
   VMware-ovftool-ver#-lin.x86_64.bundle
for the amd64 style linux installer.

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so far the technique for converting a vmware vm into a virtualbox vm using
the ovftool seems to be:

1) unpack the ovf file into a folder so we can edit the config for it.
2) throw away any manifest, since we're about to change things that it used
to know the signatures for (i.e., the ovf file).
3) remove all sections in the blah.ovf file that mention required:false.
these are not needed and will generally hose you up.
4) try to import the file as an appliance in virtualbox.  if it complains
about duplicate IDE devices, remove the cd drive.
if it still complains for devices on ide controllers, try moving the device
to the second ide controller.
if you have more than 2 disk devices, try to get just one per controller to
import, and then add the third and so forth devices after the fact.  go to
the virtualbox settings for the vm, and add them in the storage menu as hard
drives from existing vmdks or whatever.