Design layers and classes are the two most important organising concepts that you need to understand in order to create drawings in Vectorworks. I still find some users are confused about when to use a Design Layer and when to us a Class to organise their drawings. I use three simple rules to decide when I need a new Class, and three simple rules to decide when I need new Design Layers.
If you understand layers and classes fully, you will find it a lot easier to create your drawings because you know when whether should be applying a class to an object or whether you should be putting that object in a layer.
This short manual is a great way to understand when to use layer and when to use classes.