Instead of buying expensive vector drawing software, Keynote can be used as a simple but great vector drawing tool. The problem is you can only export entire slides as images from the File menu and there will be a white background. There is no obvious way of exporting just the vector art to an image file. Fortunately there is a solution. Use the following method to extract just your vector art creation to a .png file with alpha transparency.
Create a vector-based drawing.
Drag and select the entire vector drawing you are going to export.
Choose Edit