You vs. Boo is a mini-game where Mario must race against a Boo. You race Boo along 8 different courses that share the same general layout as the first 8 levels of regular Super Mario Bros. The courses bear little other resemblance to the regular levels.
Most courses feature blocks that appear and disappear depending on the alignment of switch blocks. These blocks can be switched by jumping underneath them. The switching blocks will display either a red angry face with its tongue out, which shows that blocks are out to hinder you, or a white smiling face, where the blocks are out to help you. The switch will flip from white to red at regular intervals if it is not already red, and these intervals have a set time (they are not random). It is helpful to have an idea of the timing of the flips will be so you can plan your actions.
Other courses (mainly castles) have blocks that flip between spikes and ground. The flip timing is set by red and white blocks with a countdown displayed on them. Again, it is helpful to plan your strategy so that you will have solid ground to run on when you need it. Sometimes it is even best to turn the ground into spikes early so that you can flip it back at the proper time.
Once a course is cleared for the first time, Mario's time is recorded, and the next race will have the Boo set to clear the course in this time. If Mario's time is good enough, the Boo's color will change from white, to green, to red, and finally to black. The current time for any course can be cleared by pressing START. Mario's size can also be changed by pressing SELECT, which is helpful for courses where a small Mario can fit into places better.
Boo cannot kill Mario through the course of the game, and coins do not matter. It is also possible to play as Luigi in this mode by first selecting him as the active character in Original mode.