user11304: I have a Illustrator vector shape (It has a shape path around it). But I need a mask line in the middle of the shape. The problem with with doing it by drawing a shape line over it is that I can't get the corners right.  
I want to use the middle lined mask to do animations with. 

Is there an easy way to achieve this? 

user1871: In Photoshop (or Illustrator), in a new layer above the vector shape, create a solid shape of the mask you want (with rounded corners). Export that layer alone as an image and import into AE.
Use auto-trace to create a mask from it. See https://helpx.adobe.com/after-effects/using/creating-shapes-masks.html#create_a_mask_from_channel_values_with_auto_trace

11304: <p>I have a Illustrator vector shape (It has a shape path around it). But I need a mask line in the middle of the shape. The problem with with doing it by drawing a shape line over it is that I can't get the corners right.  </p>

<p>I want to use the middle lined mask to do animations with. </p>

<p><a href="https://i.stack.imgur.com/GUVrN.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GUVrN.jpg" alt="This is what I am looking for"></a></p>

<p>Is there an easy way to achieve this? </p>
 1871: <p>In Photoshop (or Illustrator), in a new layer above the vector shape, create a solid shape of the mask you want (with rounded corners). Export that layer alone as an image and import into AE.</p>

<p>Use auto-trace to create a mask from it. See <a href="https://helpx.adobe.com/after-effects/using/creating-shapes-masks.html#create_a_mask_from_channel_values_with_auto_trace" rel="nofollow">https://helpx.adobe.com/after-effects/using/creating-shapes-masks.html#create_a_mask_from_channel_values_with_auto_trace</a></p>
