virtualhost - nginx reverse proxy and apache backend on seperate hosts - all requests go to same apache vhost

Scenario

I want to configure a dedicated nginx reverse proxy that acts as a gateway for multiple backend servers running a traditional lamp stack.

I want to make all requests for any subdomains of example.com and mydomain.com to go to the nginx machine, and send the request to the correct backend. For now i only have one backend server running apache 2.4 with two vhosts, and all requests i make to both subdomains return the content of the same apache vhost.

In a nutshell: site1.example.com and site1.mydomain.com always return the site configured in the apache vhost for site1.example.com

When i tail the access logs of both nginx vhosts i see the requests coming on on both vhosts if i request site1.mydomain.com, with seperate requests bouncing between both vhosts, so it seems something is wrong in my nginx config.

The nginx and apache machine communicate over a vlan with nginx on 192.168.1.2 and the apache machine on 192.168.1.3

Nginx config

backend_01 config file

server {
  server_name site1.example.com;


        location / {
          # app1 reverse proxy
          proxy_set_header X-Real-IP $remote_addr;
          proxy_set_header Host $host;
          proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
          proxy_pass http://192.168.1.3:80;
        }

   access_log      /var/log/nginx/site1.example.com_access.log;

   error_log       /var/log/nginx/site1.example.com_error.log;

}

server {
  server_name site1.mydomain.com;

        location / {
          # app2 reverse proxy
          proxy_pass http://192.168.1.3:80;

          proxy_set_header X-Real-IP $remote_addr;
          proxy_set_header Host $host;
          proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        }

   access_log      /var/log/nginx/site1.mydomain.com_access.log;
   error_log       /var/log/nginx/site1.mydomain.com_error.log;

}

Apache config

vhost for site1.example.com

  ServerName site1.example.com
  ServerAlias *.site1.example.com


  DocumentRoot /path/to/docroot
  
    Options Indexes FollowSymLinks MultiViews
    AllowOverride All
    allow from all
    Require all granted
  

    LogLevel error
    ErrorLog /var/log/apache2/site1.example.com_error.log

    CustomLog /var/log/apache2/site1.example.com_access.log combined

vhost for site1.mydomain.com

  ServerName site1.mydomain.com

  ServerAlias *.site1.mydomain.com

  DocumentRoot /path/to/docroot
  
    Options Indexes FollowSymLinks MultiViews
    AllowOverride All
    allow from all
    Require all granted
  


    LogLevel error
    ErrorLog /var/log/apache2/site1.mydomain.com_error.log
    CustomLog /var/log/apache2/site1.mydomain.com_access.log combined

I have tried setting a default flag in one of the servername directives in the nginx config file but this did not change anything.
The nginx.conf file is not manipulated so i don't think something there is causing this but i'll include it anyway just in case.

nginx.conf

user www-data;
worker_processes 4;
pid /run/nginx.pid;

events {
    worker_connections 768;
    # multi_accept on;
}


http {

    ##
    # Basic Settings
    ##

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;

    keepalive_timeout 65;
    types_hash_max_size 2048;
    server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;


    ##
    # Logging Settings
    ##

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    ##
    # Gzip Settings
    ##


    gzip on;
    gzip_disable "msie6";

    # gzip_vary on;
    # gzip_proxied any;
    # gzip_comp_level 6;
    # gzip_buffers 16 8k;
    # gzip_http_version 1.1;
    # gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;


    ##
    # nginx-naxsi config
    ##
    # Uncomment it if you installed nginx-naxsi
    ##

    #include /etc/nginx/naxsi_core.rules;

    ##

    # nginx-passenger config
    ##
    # Uncomment it if you installed nginx-passenger
    ##

    #passenger_root /usr;
    #passenger_ruby /usr/bin/ruby;

    ##
    # Virtual Host Configs

    ##

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}


#mail {
#   # See sample authentication script at:
#   # http://wiki.nginx.org/ImapAuthenticateWithApachePhpScript

# 
#   # auth_http localhost/auth.php;
#   # pop3_capabilities "TOP" "USER";
#   # imap_capabilities "IMAP4rev1" "UIDPLUS";
# 
#   server {
#       listen     localhost:110;
#       protocol   pop3;
#       proxy      on;
#   }

# 
#   server {
#       listen     localhost:143;
#       protocol   imap;
#       proxy      on;
#   }
#}

I have tried setting a default server and adding listen 80 to the vhosts but this does not work for me.

I also want to add that i am pretty new to nginx and might be missing something very obvious.

System info:

Ubuntu server 14.04 - 3.12.46-guest-39-a97a54c-x86_64 #4 SMP Mon Aug 10 11:59:25 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

nginx version: nginx/1.4.6 (Ubuntu)