(now diagrams.net) is the best free, open-source alternative to Visio. It supports .vsdx import/export, works offline, and offers an official portable version.
Microsoft Visio remains the industry standard for professional diagramming, floor plans, and flowcharts. The "portable" version is often sought by users who need to run the software from a USB drive without a full system installation or administrative privileges. Microsoft Visio Portable Download
Microsoft does offer an official "portable" version of Visio designed to run from a USB drive without installation. If you are looking for a way to use Visio on the go or without a full local installation, there are three primary official alternatives: Microsoft Support 1. Visio for the Web (Closest Official "Portable" Option) (now diagrams