vether vs tap [was Re: Simplify bridge(4)]

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Fri, 12 Feb 2016 21:21:29 -0500
   From: Thor Lancelot Simon <tls%panix.com@localhost>

   On Fri, Feb 12, 2016 at 10:17:31AM -0800, John Nemeth wrote:
   >      tap(4) is a direct interface between userland and the network.

   Only if you actually hook something up to the userland side.  Otherwise,
   hooked to a bridge, it is exactly and only what vether(4) is.

If tap(4) is not hooked up to anything on the userland side, why isn't
it functionally equivalent to vether(4)?  What is different in the
idea of vether(4) that would make <https://gnats.netbsd.org/40139>
work?