OK try this, you didn't specify which version of Apache you were using and you seem to have Apache 2.2 syntax mixed up with Apache 2.4 syntax so I have given both versions.
Change this section back to how it was originally, this controls access to your C:\
and you just allowed full access to it, NOT GOOD.
From
<Directory />
AllowOverride All
Options All
Require all granted
Order allow,deny
</Directory>
to Apache 2.2.x syntax
<Directory />
Options FollowSymLinks
AllowOverride None
Order Deny,Allow
Deny from all
</Directory>
Apache2.4.x syntax
<Directory />
Options FollowSymLinks
AllowOverride None
Require all denied
</Directory>
Now to your Virtual Hosts. This also need to have it's own security specified inside the block
Listen 81
<VirtualHost *:81>
ServerName rti.etf.rs
DocumentRoot "C:/wamp/www"
DirectoryIndex index.php
#### Apache 2.2 syntax
<Directory "C:/wamp/www/">
AllowOverride All
Order Allow,Deny
Allow from all
</Directory>
#### Apache 2.4 syntax
<Directory "C:/wamp/www/">
AllowOverride All
Require all granted
</Directory>
</VirtualHost>
PS. I cannot see any benefit in using port 81 and it just makes life more complicated for an external user.