I wonder what leads you to your assumptions. Maybe a bit more reading around would have shown you that each VM (like VMware, VBox or QEMU) has a different (default) format for the (virtual) disks. All of them have the same purpose: to emulate a "real" hard disk of a computer.
The (default) VBox disks are those .vdi files, but VBox can also handle .vmdk files straight away. Furthermore there are tools available that can convert pretty much any of these formats into any other one. But that would only be required if you move from VM to another one and have a good reason for a conversion.
For example I'm also using a Ubuntu VM (under Vbox) that uses a variant of a .vmdk file to utilize a raw image file created by WUBI which is usually loop-mounted when booted directly. But I consider this a bit more advanced stuff, so don't worry if you did not quite get my setup.
Furthermore there is not much what can go wrong when installing in a VM. You won't screw up your host system. That's the whole point of a VM (at least in my view). It allows you to experiment wildly and if you've stuffed it up you go back and try a different way. All this without risking your host.