As you probably noticed using your existing USB install media on a new Windows 8.1 computer is that it doesn't show up in the boot menu without enabling Legacy support. Why you ask? Well the EFI bios doesn't support NTFS only fat32 and the USB stick also has to be partitioned with a GPT partition not MBR.
Luckily there is a tool which does all of this for you. It's Called
http://rufus.akeo.ie/ <----- Download link
Insert your USB stick, for a Windows 8 install stick you will want one about 8GB in size. (it won't fit on a 4GB stick)
1) Under Device select your USB drive
2) Under "Partition scheme and target system type and select "GPT partition scheme for UEFI Computer"
3) Under File system select "Fat32"
4) Under Cluster size use the Defualt
5) Give your USB device a name
6) Check, Quickformat
7) Select your Windows 8 ISO (Not sure about vista) that you wish to use
8) Click Start
That's it! It will now create a Bootable EFI a compatible USB drive.