I have been using Inkpad for vector graphic illustrations since the early days of iPad and have been quite impressed! It is the easiest to use if the many apps I have evaluated, and its drawings export nicely to most note taking apps.
I was worried because no updates came along for a long time. But right after iOS 7 was rolled out, an iOS 7 update to Inkpad appeared. Joyous, happy days! There was apparently still life in this great app!
Recently, another update appeared which included arrowhead endpoint embellishments on straight lines. A wonderful improvement!
If you need a vector graphics drawing app, go get it right now! It is fabulous? (I don't say that about many apps, and please understand that I have no personal connection with the developer. I am just a satisfied user, and I would like to see the app continue to be improved.)