You don't have to move anything although some people do just to allow some circulation under the rocks, etc. Vacuuming around plants can be tricky. It really depends on what type they are and how heavily planted the tank is. The more plants, the more difficult it is to do a complete gravel vacuum. On the flip side of that, the more plants, the less likely the waste that you don't get out is to cause a problem with ammonia or nitrite as the plants will help process and use it.