Hi Atul --
It sounds like you're assuming that Tableau's httpd access log uses Common Log Format - which would include the username.
If you're after the username logging in (both successes and failures) you should look at the vizportal-*.log files instead. You'll find them /data/tabsvc/logs/vizportal.
Hope this helps!
Thats definitely correct.
I did found it later on while checking other logs.. Forgot to close this thread!
Anyways Thanks for the same