If you are able to boot VWware through a USB stick when creating or testing a bootable USB drive, you are lucky because VWwareand virtualBox do not support USB naturally. But for everyone else, with free software known as Plop, you can boot them from a USB drive. When Plop prompts, you have to load the programs from USB or a hard disk, providing that you have set up the BIOS. Plop works with VirtualBox and VMwar. It's not mandatory to load the operating system for a virtual machine; having an "empty" virtual machine is ok. After downloading the Plop Boot manager, save by uncompressing the ISO logo to the hard disk.
The VMWare virtual machine's BIOS does not offer the option to boot from USB. It's quite a disadvantage because this is very convenient for testing - for example - the Linux distributions on a USB key.