in your browser you can just type in clientB.localhost.com and you're all set. So here's how to do it in 30 seconds on windows
First thing we need to do is edit our hosts file to tell it when you type in that address where to go so open up in notepad:
C:\Windows\system32\drivers\etc\hosts (may be different on your system)
add these lines and save the file
now open up the httpd.conf file in C:\Program Files\Apache Group\conf\httpd.conf
and find the VIRTUAL HOST section. (search for "VirtualHost example:")
add add these lines
save that file and restart apache, now open up your browser of choice and type in clientB.localhost.com and boom you're virtual.