virtualbox.org

I have a bootable flashdrive, with an operating system on it. It boots on my pc. I want to be able to use it in virtualbox as well, and sort of do now, but this invovles ripping an image of the flasdrive, making that into a vhd, then reversely, reflashing the drive with the image. This is if I want to make any changes to the flashdrive in virtualbox. I'd like to be able to boot the usb drive directly in virtualbox, as the vhd. Is this possible?

Only by mounting the drive as a raw disk (a very dangerous feature), as VirtualBox VMs can't boot from a USB drive.

Is it even possible to mount a physical hard drive as a VHD?

AFAIK only VMDK allow you that.

If you already have a "VHD" file you don't have a physical disk. You have a large file on the disk and it is called *.VHD.
All you need to do is create a guest in VirtualBox and tell it to use the VHD file. Other than that you use the raw image like mpack said and setup a raw disk in VirtualBox.
This requires expert level knowledge though and you will find very few that will want to help because it is so dangerous.