You only need to vacuum the gravel where there are no plants. The plants will use whatever mulm falls into the substrate near their roots. If your substrate is completely covered with plants then you don't need to vacuum the substrate, just get anything loose that's on the surface. If you don't already have some, try to get some malaysian trumpet snails, they will burrow in the substrate and prevent anaerobic pockets from forming.
Edit: this doesn't apply to plastic plants, plastic plants are just decoration, so treat them like any other decoration and vacuum under them every now and then. As the previous poster mentioned you'll still have to do water changes in a planted tank.