A virtual directory is a link to existing physical directory that is present on the server's hard disk. Virtual directories can have a number of specific settings like custom ASP.NET configuration, access permissions, URL password protection, and so on.
Virtual directories can also be used as aliases. For example, you have a web application installed on your domain 'yourdomain.com' in the physical folder '/my_data/web_apps/forum'. To access this web application, users need to type 'yourdomain.com/my_data/web_apps/forum', which is hard to remember and too long to type. You can create virtual directory 'forum' in the root of your virtual host, and link this virtual directory to '/my_data/web_apps/forum', so users who want to access the web application have to type 'yourdomain.com/forum' , which is much shorter and easier to remember.
To create Virtual Directories, Login to Plesk Control Panel and go for the Website & Domain tab, after that click on Show Advanced Operations then select Virtual Directories.
Now click on the Create Virtual Directory icon.
Specify the required parameters to create a Virtual Directory.
Name - specify the virtual directory name.
Enable the permissions.
Select the Execute Permission.
After all these steps just click OK.
New virtual directory has been created.