We'll likely know either way soon, but "Vesuvius" feels more like a placeholder name for the supposed expansion (assuming it's legitimate). Whether it's a Natural Wonder or in reference to the disastrous eruption, it would most likely only be one of several; it'd be like naming an entire expansion with eight or ten Civs "The Ottomans" because they happen to be included, for example. Just my personal thoughts on the topic.
Whether this hours-long video of various weather and clouds might indicate some form of weather pattern mechanic for Civ VI (which would be difficult to simulate in a game whose turns represent entire years, decades, or centuries) or hints at some sort of natural disaster scenario mechanic is impossible to say, but I know one thing: I'd personally MUCH prefer a 30-second teaser video that says "Watch This Space" or "Tune In At Such-And-Such-A-Time" to what Firaxis is doing with this "promotion". Sure it's drawn a lot of attention, but the viewership has been dropping steadily for hours now. They just need to come out and say what it's about.