Here is a completely FREE vector icon of the instagram app.  I was unable to find a suitable vector logo so I decided I might as well make my own and provide it to everyone to use.  So this file contains a .ai file, .eps file, and two png files at 300px x 300px and 30px x 30px.  The .ai and .eps file will work in adobe illustrator CS3 and higher or other vector based program.  Click on the image to download the file or else you can click here.  Enjoy!!