What needs to happen for him to sew it up at Vegas?
He's currently 62 points ahead and Lando has 75 regular race points available, 8 sprint points and 3 potential fastest laps. So theoretically still enough to overhaul him with very favourable results.
After Vegas there will be 50+8+2 (60 total) left available.
So Lando needs the gap to be 59 points or less after Vegas for it to be mathematically still possible to catch Max - so as @Stumble Bum says, if he outscores Max by 3 points, then mathematically it's not done and dusted yet
To look at it from the other perspective, lets say Lando has a virtually faultless return of 3 wins and a sprint win (but not fastest laps, lets assume RBR throw softs on Perez last laps etc.) - he'll sit at 414 points, which is just 21 points more than Max currently has - so as long as Max can average more than 7 points per weekend from here, he wins regardless of how well Norris does.