That is really awesome if you are drawing on your iphone. I find the screen too small to draw anything significant. Making vectors is good because you can do them with a mouse, but having a tablet makes it slightly easier and faster to do.
I couldn't do my drawings at first. I took some time to learn the program so that I could do the drawings. You won't get it right away but with some patients, practice and a good snack, you can do it. I actually made it in adobe Illustrator.
I would not recommend GIMP for vectors. Inkscape(free), Illustrator or Photoshop(if you can afford it) are the preferred programs for vectoring. But useing what you know is good to just need to practice. Thank you!