I was in a game the other day, playing bot lane with my friend. We got ganked by a Jarvan, who proceeded to place a flag behind us and try to dash to it. But, due to my epic skills and reaction time I landed an E and threw him off of us before his dash finished (totally a lucky shot). I was wondering if Thresh's E can also cancel other dashes like that one. To give a few examples, Vi's ultimate, Malphite's ultimate, and Maokai's Twisted Advance?
Yes. But, some dashes make the person dashing unstoppable or untargetable. All of the examples you listed make the dasher unstoppable or untargetable. Vi's ult and Malph's ult both make them unstoppable, and Maokai's W makes him untargetable. You just have to memorize whose dashes do what. Dashes like Vi's Q and Lucian's E can be interrupted by abilities like Thresh's flay.