Keeping a swimming pool clean enough for swimming is a lot of work. An important part of this involves periodic vacuuming of the pool's bottom. Swimming pools are bodies of water open to the air, and all manner of things, including dirt, can fall into them. If you're vacuuming your pool to eliminate dirt and it keeps returning there are a couple of reasons why, including poor filtration.
Swimming pools act as traps for anything carried on the wind or rain. Dirt, bird droppings and algae often end up in a pool, where they become particulate matter. Ideally, particulate matter that ends up in pool water will be filtered out. Some pool particulate matter can settle at a pool's bottom, where it'll need to be filter vacuumed. Unfortunately, a pool filter that's been backwashed (reversed) prior to vacuuming a pool often puts dirt back into the pool.
If dirt is reappearing at the bottom of your swimming pool after you've vacuumed it your pool's filter may be working poorly. Pool filters often work poorly because they're in need of cleaning. If you have a sand filter for your pool you need to make sure that the sand is sharp and freshened up. Use a commercial pool filter cleanser occasionally and backwash your pool filter monthly to free it of any trapped dirt or other particulate matter.
Swimming pools that end up with a lot of dirt in them are typically under assault from windy conditions. High winds or winds that blow constantly often carry fine particulate matter like dirt. If you're vacuuming your swimming pool in the early morning and dirt is back in the pool later that day, consider purchasing a pool cover.
If your swimming pool has a large amount of dirt try vacuuming it using the pool filter's "vacuum to waste" setting, if it has one. This bypasses the filter unit, sending the water to the pool's waste line. Lastly, run your pool's filter at least 6 to 8 hours daily in order to thoroughly filter the water. The longer you can run the filter, the better.