If a teams starts running the slot vertical I've done a few different things in the past.
1. Cover Four is one of the best basic coverages for stopping vertical routes because the Safeties sit in the passing lane where the slot will run his route.
2. Man coverage. Jam the slot (preferrably not with a linebacker but with someone that can stay with the receiver). Or you can just play tight man coverage. The best coverage in principle for defending the pass is man coverage because you know there's a defender with the receiver.
3. As far as a technique or twist you could employ, I love using Robber coverage when a team starts getting pass happy with their slot receiver or start running their #1 ("X" receiver) on a post or deep dig route behind the backers and between the deep thirds in cover 3 or deep middle in cov 2. Robber basically tells the safety to read his keys to determine run or pass. If he reads run he fills the run lane. If he reads pass then he rolls down to read the #2 route (second route from the sideline) which is your slot in the example provided. If the slot goes vertical then he is the #2 route from the sideline and the safety runs with him. There are several options and rules you can use if the #2 breaks his route in or out depending on how many second level defenders you have in zone coverage.
Robber is more of a helper on the #2 route, but to simplify it by using your example, a go route by the #2 would be the 2nd route from the sideline. If twin receivers run a post/wheel or a slant/wheel, then your "X" receiver (#1) then runs the #2 route. In this case, the safety is going to "Rob" the #2 route of the outside receiver as he breaks in.