It is surely not in VCL. However, do note that it's much better to start building from the provided default.vcl, adding bits by bits, to understand how it works.
As for why it doesn't work and ways to fix it.
Different ports (duh).
Option #1. Tell your PHP what the real port is.
Perhaps, the most easy fix would be to edit wp-config.php and tell the Wordpress where the request really came from. Add at the top of the file right after <?php:
$_SERVER['SERVER_PORT'] = 80;
Option #2. Just use the same ports.
You can actually use same port number(!) in both Varnish and Apache. Simply bind Varnish and Apache onto different interfaces, but same port numbers. To a Linux machine those are different ports, and there is no issue using same port number.
So Apache will have:
Listen 127.0.0.1:80
Varnish will have in the VCL:
backend default {
.host = "127.0.0.1";
.port = "80";
}
Option #3. Don't use Apache. Nginx to the rescue.
A not so easy fix perhaps would be switching to Nginx which can easily circumvent the "port in redirection" problem via simply one line of configuration:
port_in_redirect off;
Wordpress base URLs.
If the Wordpress site URL (in wp-admin) is set to include port number, then naturally that's where it will redirect you. No port number should be present in Wordpress site URL.
Bonus Tip: Fix Wordpress Jetpack in Varnish.
