vWii (or Virtual Wii) is the fanmade name for the emulated Wii mode of the Wii U. It uses modified IOSes and a modified Wii System Menu to account for changes in the Wii U's hardware and the virtualized environment which is booted directly from Wii U mode using a technique called cafe2wii.
System Menu encryption
The vWii System Menu uses a custom encryption not present on the original Wii; this makes it impossible to install modifications such as Priiloader without breaking the encryption first.
vWii lacks the Wii's 3 bootloaders (boot0, boot1, and boot2); instead, the Wii U launch code (cafe2wii) directly launches vWii IOS and the System Menu. This can be used to boot into a custom title other than the System Menu such as a particular channel or homebrew app.