I am trying to learn Ninja Vanish and the loop forms but I can never get the yoyo into the loop! The loop is always going too fast for the yoyo to land on the bottom string. HELP
If you can form the loop then you can do the trick. It takes a while to get consistent. Exaggerate the motion to make the loop larger, and eventually you’ll get the motion down and know how much to “whip” it and you will be able to do it with your eyes closed 
Actually I really don’t understand the motion. What is the correct method for doing this trick
Ah yes, this puzzled me when i started out as well. It all happens so fast it’s hard to tell… But if you watch a slowmo version, i think you will understand. Its really hard to explain in words and you probably wont understand but I’ll try my best. So you throw your breakaway or sideways sleeper, whatever… Now you pinch the string about a 3rd-4th way down. Like in the other tutorials what you are trying to do is whip the string and get a loop to form and land into a green triangle. So when you are “whipping” the string you pull a little towards yourself. That way the string goes over towards your body and forms a loop. You might want to pull the string a little to the right to open up the loop. So you pinch, whip or pull the string towards you so the slack goes towards you and goes over the string to form a loop. Once you can get the loop open, exaggerate the motion to increase the size of the loop, the yoyo should drop and land on the string to form a GT, you will have to experiment a bit at first to see how much to whip it… If you still dont get the motion put the yoyo in your lap sitting down, and just whip it while the yoyo is on your lap so you can see what the string is doing. Move the string around to to get the under the yoyo as its dead, basically just experiment with the moteion while you are sitting down so gravity isnt pulling the yoyo down. This is what i did to understand whats going on and how to do it. After that i just practiced until i got the feel for it. This trick happens very quickly, so make sure you understand how to do it and what is going on. Hope i helped
What I think would be easiest to get the motion down easier is starting out with the string pinched and your palm facing up, then whipping and turning your wrist over to form the loop better. This also helps sort of guide it into the gap. 
“It’s all in the wrist.” - Happy Gilmore
That seems to be the key part for the loop and landing it for me. Pinch with the finger and thumb palm up. Pop the yoyo up and flip your palm over with the wrist until your thumb and forefinger are pointed straight down at the floor and palm is now facing left. Once you get the motion down you can do it quick or stall it out and really slow it down!