Said about a plan, a system, a product, a team, a skill — something that people have praised loudly and often, usually too loudly. This is not a neutral word. When a writer says "the vaunted new security system," they are almost always about to tell you it failed, or that it is not as special as everyone said. The praise is in the word; the doubt is in the writer's voice. So it works like a quiet eye-roll: "the much-vaunted reforms," "their vaunted defence." You will see "much-vaunted" together very often — that pair is almost a fixed phrase. It belongs to newspapers, sports writing, business writing and reviews, not to everyday conversation; a Taiwanese student is far more likely to read it than to need to say it. Note that the doubt is not built into the dictionary meaning by force — you could in theory use it as straight praise, and older writing sometimes does — but in modern English the sneer comes with it so reliably that if you use it warmly, readers will still hear it coldly. There is also a verb, to vaunt (to boast about something), but it now sounds old and bookish; the -ed adjective is the form that survives in normal reading.