Masking Help: How do I mask a shape constrained to the pixels of another shape

[AspiringPerson]

This might be silly question but I can't seem to figure out how to do this, with Designer or Even Illustrator. 

I have a layer with a shape on it called "Blend"

I want to draw vector elements and have it be constrained to the pixels on the blend layer. Really what I want to is use vector elements to subtly act as grain, but I only want it on that part of the project. 

I've tried adding a mask layer, adding a empty mask layer and copy and pasting the content into. Nothing seems to work. It seems like something that should be really simple so I must be missing something. 

[DM1]

This explanation may help.

16 hours ago, AspiringPerson said:

I want to draw vector elements and have it be constrained to the pixels on the blend layer. Really what I want to is use vector elements to subtly act as grain, but I only want it on that part of the project. 

 

[AspiringPerson]

@DM1 Thanks that's a good explanation