Hey Darth, when you have your project open you can press ALT + ENTER (assuming you are on windows) and this will open up your project settings (if you're on Mac it'll probably be in one of the menus). When you are in the project settings window you should see a drop down menu 'Frame Rate' and you can type in 15 there so the project will become 15 FPS.
I gathered from your post that you are importing pictures from your camera into Vegas and making video files with Vegas? Now this way works but you have to set the picture duration to 1 frame at 15fps. I know you said it only lets you choose "x frames at 29.970 FPS" but now that the project is set to 15 fps it will give you the options for 15 fps
I hope this works for you!