Create middle lined mask in vector shape
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