How do I add a vector repeating pattern to a shape?

[Staren]

I have what I hope is a straightforward question, but I can't find an existing answer. 

I want to fill a shape with a repeating pattern and have it stop at the edge of that shape. I'd like to take the below pattern and drop it into the red area on the other image without it going outside the shape or converting it to a bitmap. The pattern has to stay a vector. It cannot be converted to a bitmap. The file will be used to create a cutting die and the vector lines are necessary, so a raster image anywhere on the canvas will not work. 

I cannot seem to find a quick way to do this. Does anyone have a process? 

Edited May 8, 2023 by Staren
Added images for illustrate issue

[firstdefence]

1. Create the pill shape in vector form using the rectangle shape
2. Duplicate the pill shape you have just created and use the contour tool to get the inner red pill shape (Bake Appearance)
3. Create the diamond pattern using the diamond shape and utilising power duplicate (CMD + J) and group or merge curves.
4. Nest the diamond pattern under the red inner shape.

 

[Staren]

Of course the answer is that simple. I could have sworn I had tried that, but the pattern clipped instead of masked. I must not of moved the layers correctly. Thank you for taking the time to run though the steps.