Use a miter saw that can cut at through a 9" plank in one cut. Make sure you are using a blade designed for cutting plastic.
Typically, vinyl planks can be cut with a utility knife. You score the wear-layer, and then bend them until they snap. While this can be done with Modin Rigid Plank, the 40 mil (1.0 mm) wear layer and the rigid core makes this more challenging. You will want to use a straight edge, and push the utility knife as deep as possible into the wear-layer. If you don't get a deep enough score in the wear-layer, it won't snap clean, and you will end up with a jagged edge.
A much faster and more precise solution is to simply use a miter saw. Use a saw that can cut the full 9" width in one cut (you may need to get one with a slide). Make sure you are using a blade designed for cutting plastic. As always when using a miter saw, use appropriate respiratory protection, and attach a vacuum to minimize airborne dust.