Problem Setting First Vertex (AE)

You can have problems using the selection tool to select vertices. This method always works:

1. Make sure the pen tool (g) is active
2. Select the mask path in the timeline panel
3. Hold down the Ctrl/Cmnd key to bring up the white selection pointer and drag a selection around the vertex you want to use as the first vertex
4. Use the menu Layer>Mask and Shape Path>Set first vertex
5. Set a mask keyframe for the path
6. Cut the mask keyframe using Ctrl/Cmnd + x
7. Add Write the Write On effect
8. Set a keyframe for Brush Position
9. Paste (Ctrl/Cmnd + v) to paste the mask path to brush position

If the path goes the wrong way you can Time Reverse keyframes by selecting the Brush Position property in the timeline then using the menu Animate>Keyframe Assistant>Time Reverse Keyframes.

FYI, I almost never use Write On. I use a shape layer path with Trim Paths added. You have a lot more control and a lot more options. Just do everything listed above but instead of adding the Write On effect, deselect all layers, use the pen tool (g) to set the first vertex anywhere in the comp panel and paste (Ctrl/Cmnd +v). This will give you a shape layer path in the same position as your mask providing the transform properties of the layer with the mask are all at their defaults. Now you can press the U key twice to reveal all modified properties of the path, change the stroke width, end caps, dashes, gaps, and then add trim paths to animate how the stroke draws on. The shape layer can then be used as a track matte. It's a much cleaner solution that retains the ability to continuously rasterize and add other shape layer animators.

I hope this helps.