Thank you for the support topic, it seems the vantage themes are not AMP-compatible it uses custom javascript to toggle the menu, the custom javascript is not allowed on AMP so the AMP plugin removes them to create valid AMP pages.
You may contact your theme author asking them about AMP compatibility.
I will recommend you to switch to Reader Mode and choose different themes for AMP pages ie. your site will have your normal Vantage theme but AMP pages will have compatible themes such as Astra or Neve
You can learn more about Reader Mode in this documentation or check out the video below