From VirtualBox VM (.vdi file) to a USB bootable ISO image

I have set up a virtual machine with Xubuntu Linux in VirtualBox.

The disk is 4.5 GB a .vdi file.

How can I make an IMG or ISO file from this, such that I can then write it into a bootable USB flash drive,

?

TL;DR: I’m searching for a convert-vmdisk myvm.vdi myvm-bootable.img tool.

I’ve already tried answers from Is it possible to convert virtual machines to physical environments? and especially

VBoxManage internalcommands converttoraw file.vdi output.img

but then after writing this .img to a USB flash drive (with Rufus), the PC won’t boot on this USB flash drive.

Linked topic: forums.virtualbox.org/viewtopic.php?t=40124