It's not going to be as vivid as something you see in broad daylight with your two eyes wide open. But it will
get (a lot) more vivid, with practice.
Okay, you try something like this, play around and see what happens. Close your eyes, relax, breathe etc etc, then pull up an actual memory. For example, the memory of the house you lived in when you were a kid.
Now, an image will pop into your mind. It could be something like the way your house looked, from the outside. Hold the image gently ... gently .... gently.
Now start visualising it in greater detail. The colour of the walls. The colour of the door. The design of the gate. The position of the tree in the garden, relative to the rose bushes. The texture of the pathway leading to the front door. The bicycle on the right hand side of the porch.
Now ........ walk through the front door .... and go through every room. The hall, the kitchen, the dining room, the bedrooms. Pay attention to where everything is, slowly fill the image with details of things that you recall were really there in your old house .... the wooden floor, the green sofa on the wooden floor, the square cushions on the green sofa, the cat sleeping on the square cushions .... and so on.
Okay .... that was visualisation practice. Here you're working with an actual memory, a place that really existed, a place that you knew well.
Now ... for purposes of LOA/IM, it's the same thing but you don't have to work with an actual memory. You just visualise whatever you want (eg your desired dream house), and start filling in the details - as you had filled in details for your actual house.
With practice, (and I emphasise - not immediately, but with practice), this can become a very vivid kind of experience. It's not limited to "image" purely ... it will be 3D, you can fill it with sound, touch, movement, even taste ....
..... eventually go a few steps further, sink really deep into alpha, and you might meet a couple of walking, talking entities in your visualisation exercise. (like Napoleon Hill's counsellors).
While furniture, houses and other inanimate objects in your visualisation exercise will generally appear and behave exactly as you want them to, don't be surprised if the walking, talking entities DON'T. You can make them disappear (or you can just "wake" up and end the visualisation) but you can't order them around to do things. They have their own free will.
If you meet a bunch of them and they call themselves "Abraham", let me know. I'd love to meet them too.