Why not use http://www.autotracer.org/ with the better image you have?
or if you don't want go to the Net use the free Expression 3 (old creature house) who has a tracer inside! :)
http://www.microsoft.com/en-us/download/details.aspx?id=31249

So you can have real curves inside moi ;)

As you can see you have interest to have the more clean start image possible!
Here with any regulate!
I have just enlight in yellow some curves but all is real curves directly inside MOi! (here from EPS format from autotracer with regulates by default!