VMDK is using to actually reach the purpose of the task you have asked : Exporting. If you export, it means it leaves your system, your known environment, you ship it somewhere else, somehow, to be used by something.
VMDK is the only widely format support, so it only make sense to export to VMDK. I don't see any issue with that.
To me, it only appears that you are using the wrong tool for the job : if your VM will only run on Virtualbox environment, why would you use export in the first place? You are not leaving your known system, you are only moving your data accross.
If you zip/tar up your VM folder, you're basically doing the same thing as exporting, without the hassle of transforming to VMDK.
This also tell me that this is a duplicate of
your other post, under which you end up with the same logical conclusion as me from mpack.
So right now, this present post is off-topic, while your other one is either a bug report, or a suggestion - so in the wrong forum.
Either way, you don't gain anything in exporting in the first place, only loosing, so why insisting on it?