??? Nop, I do the basic shapes on top of the picture.
Here's the process I use:
1. Get the idea
2. Get a good stock photo for model, I try to get huge pictures.
3. In photoshop draw with the pen all the basic shapes of your object in top of the picture. The key here is to make it simple and get just the BASIC shapes of your object. For example the car (that as the most vector shapes I think) as around 7-8 vector shapes that's it. The details will come later
4. Work each part separately. Work colors, lights and details
That's pretty much it.
I'll say the plane is 20h of work, the bus is like 8-10 ad the car is 2-3h