Although not offline, the online version of Draw.io (app.diagrams.net) requires zero installation. Just open a browser, create your diagram, save to cloud or local drive. Works on locked-down school or corporate computers where USB execution is blocked.
If you specifically need a standalone app that runs from a USB drive (true "portable" software), consider these powerful, free alternatives that support Visio files: Visio Portable Download
Once you have downloaded Visio Portable, you can use it on any computer by following these steps: Although not offline, the online version of Draw
Can open some .vsdx files but may lose advanced shapes. Although not offline