Multiple Material Collection Parameters

I’ve just set up a Material Parameter Collection which uses a Location vector to offset my grass a bit. The effect is that the grass moves as my character moves through it.

The issue now is to make this work with multiple characters.

I’m pretty sure that I need to add multiple vector parameters:

How can I do this via blueprint or c++. I think it would happen in my character BP Begin method right?

The other thing is how do I iterate through all of the vectors in my grass material?

Thanks!!