We draw vector diagrams to simulate what we see in the real world. I think some people still don't get this.
As I see it, there are two main rules when drawing vector diagrams:
1. Join your vectors 'tip to tail'
2. Use your common sense!
If the resulting velocity, displacement or force looks wrong then use your common sense. Th diagram is only showing you what will happen if you put these two velocities together or these two forces together.