I am currently using nginx reverse caching proxy with apache and w3 total cache and self cdn with generic mirror to 4 subdomain. The site works well but has moments when it hangs for 10-15 seconds. I'll post the nginx conf and vhost conf to check if everything is correctly set.
Is there any point in using varnish with this configuration?

When you're using a reverse proxy you don't really need tools like W3 Total Cache. What they essentially do is generate static pages with are faster to deliver, more or less exactly the same as a reverse cache does, only it stores them in memory and request pages that don't already exist in cache from the backend server.

There is little point in using Varnish when you have configured Nginx as a reverse cache. Varnish do however have really good cache invalidation mechanisms.

I bet the hang you're seeing is W3 Total Cache generating new pages every now and then. Using that many subdomains might actually slow things down given all the DNS requests that has to be made in order to start downloading assets. At least something to think about.

Thanks for the answer. I arrived home, so I can post my configs.
The apache vh conf(/var/apache2/domain.com.conf) file looks like this:

<VirtualHost 127.0.0.1:81>
     ServerAdmin admin@email.com
     ServerName domain.com
     ServerAlias www.domain.com
     DocumentRoot /var/www/domain/
  <Directory /var/www/domain>
      Options Indexes FollowSymLinks
      AllowOverride All
      Order allow,deny
      allow from all
   </Directory>
     ErrorLog /home/user/logs/error.log
     CustomLog /home/user/logs/access.log combined
</VirtualHost>

Then I installed nginx and set the nginx.conf like this:

user username; #change to the same user apache runs as
worker_processes 1; #change to the number of your CPUs/Cores
worker_rlimit_nofile 8192;

error_log /var/log/nginx/error.log;
pid /var/run/nginx.pid;

events {
  worker_connections 1024;
  use epoll;
  accept_mutex off;
}

http {
  server_names_hash_bucket_size 64;
  include /etc/nginx/mime.types;
  default_type application/octet-stream;
  access_log /var/log/nginx/access.log;
  sendfile on;
  tcp_nopush on;
  keepalive_timeout 65;
  client_max_body_size 100M;

  # reverse proxy options
  proxy_redirect off;
  proxy_set_header Host $host;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

  # gzip compression options
  gzip on;
  gzip_http_version 1.0;
  gzip_comp_level 6;
  gzip_min_length 0;
  gzip_buffers 16 8k;
  gzip_proxied any;
  gzip_types text/plain text/css text/xml text/javascript application/xml application/xml+rss application/javascript application/json;
  gzip_disable "MSIE [1-6]\.";
  gzip_vary on;

  # include virtual hosts configuration
  include /etc/nginx/virtual.d/*.conf;
}

After this, I created the domain.com.conf file in /etc/nginx/virtual.d that looks like this:

proxy_cache_path  /var/lib/nginx/cache  levels=1:2   keys_zone=staticfilecache:180m  max_size=500m;
proxy_temp_path /var/lib/nginx/proxy;
proxy_connect_timeout 30;
proxy_read_timeout 120;
proxy_send_timeout 120;

#IMPORTANT - this sets the basic cache key that's used in the static file cache.
proxy_cache_key "$scheme://$host$request_uri";

upstream wordpressapache {
        #The upstream apache server. You can have many of these and weight them accordingly,
        #allowing nginx to function as a caching load balancer 
        server 127.0.0.1:8080 weight=1 fail_timeout=120s;
}
server {
  listen 80;
  server_name domain.com www.domain.com;
  access_log off;
  error_log off;
  location / { proxy_pass http://127.0.0.1:81; }
  location ~* ^.+\.(htm|html|jpg|jpeg|gif|png|ico|css|zip|tgz|gz|bz2|pdf|odt|txt|tar|bmp|rtf|js|swf|avi|mp4|mp3|ogg|flv)$ {
    expires 30d; #adjust to your static content's update frequency
    root /var/www/domain;
  }
}

Which guide would you suggest me to follow to install Varnish?

Just installed Varnish and I am satisfied with the performance.I am running the apache with nginx reverse proxy cache and Varnish. I disabled W3 Total Cache and some useless plugins. Noe I try to setup a subdomain for all images to send multiple requests.
On homepage it takes approx. 0,600 sec to call 160 queries. I guess it's good, Tell me if I am wrong.

You don't really need both Nginx and Varnish in your stack, since you've configured them to do the exact same thing. You can however replace both Varnish and Apache with only Nginx and still have a reverse cache, but I'm not sure if that is within your current scope.

600ms just in the database isn't awful, but it's far from good. But as @zemadeiran has pointed out, you don't really need to worry that much about it when you're using a reverse caching proxy. Say you have a 95% cache hit rate only 5 out of every 100 requests will end up on your backend server.

I came back with another question. With this stack ngnix/varnish/apache, how should the requests be made?
Apache listening on port 8080, nginx listening on port 80 with proxy_pass to varnish on localhost:6081, and varnish with backend to port 8080?

Again - you don't need Varnish when you've configured (as you have) Nginx as a reverse caching proxy.

You want all traffic to first hit your reverse cache then your backend servers. So, Nginx (80) -> Apache2 (8080).