Performance of USB3 sticks is more than adequate and this might be a way to create a way to create a single stick that could run multiple operating systems from bootup without needing any host storage.
You can just write a filesystem to a USB device, or you can partition it and write to it like it was an HDD. So putting multiple operating systems on a USB stick has always been not just possible, but trivial; you do it just the same way as you do it on a HDD.
I made a go at rolling my own with Ubuntu, but because I trying to do it with an older version of VMware workstation running under Windows it seemed to hose up on the USB stick installation.
Get vmware player and the gparted CD ISO, and you will have all that you need to accomplish your goal. ;) Make sure to set the boot order before installing anything because it's much easier to get into the BIOS then. Can't you point VBox at a physical drive, though?
There might be too many gotchas in terms of hardware drivers for the host Linux environment, but it seemed like a sweet solution if would work.
Once you manage getting both nvidia and ati drivers installed at the same time, the rest is child's play.
If you've got a fat USB stick, I advocate installing some lightweight edition of Ubuntu to do the job you're trying to do, perhaps lubuntu. I've never tried pointing vmware at a partition, only at an actual raw device. That worked well as long as I made it an IDE device. This is on an Ubuntu host and using a SATA HDD, and later an SSD. If I told it that the disk was SCSI or SATA then Windows 7 got confused. Telling it that the virtual disk was IDE and pointing it at my SATA disk worked great.
The question I have, though, is why not just use vmware player? It costs the same as virtualbox. Last time I checked, it was vastly superior. It doesn't involve Oracle. Seems better all around.