Hi, ar5iv creator here.

You can think of both arxiv-vanity and ar5iv as the "alpha" experiments that lead into the official arXiv "beta" HTML announced today.

Once a few rounds of feedback and improvements are integrated, and the full collection of articles acquires HTML in the main arXiv site, ar5iv will be decommissioned.

The plan is to turn all existing ar5iv links into redirects to the official HTML, and free up the resources for maintaining it. I am not sure what are the plans for maintaining arxiv-vanity, but I suspect they may head down a similar path some time later.