Personally, I define vigorous exercise when I hit 95% of my maximum heart rate (220-age) non-stop for 30 minutes. 100% is doable but a bit too extreme. I think I've seen similar definitions in health literature.
Worth noting that the various "max heart rate formulas" are just guidelines. Plenty of people routinely hit above their predicted rate, and plenty of people cannot get there.
Yeah, the "based on age" makes it tough. I would love for there to be something more prescriptive. That said, I suspect just trying already puts me in a decent position.