“Do you have to change your vibration on a particular subject in order to let it in?” No, you don’t. You could pet your pet and let it in.
You could sit with your feet dangling in the bay, and let it in. If it is a subject that you often think of in an attitude of resistance, it is really worthwhile reaching for some thoughts that feel better.
You could launch an intention and never think about it again, and the Universe would yield it to you. You don’t have to clean up your vibration relative to anything, if you can just not think about it any more.
That’s why we teach meditation. It’s easier to teach you to have no thought than to have pure positive thought.
When you quiet your mind you stop thought; when you stop thought you stop resistance; when you stop resistance—then you are in a state of allowing.