Redirect Subdomain to Subdirectory without changing URL multiple WordPress

I have my folders in public_html like this,

/ - root public html for example.com [has wordpress in it] /a/ - this has another wordpress installed and a.example.com should be pointed to this directory /b/ - again wordpress for b.example.com

I tried this code,

RewriteEngine On
RewriteCond %{HTTP_HOST} ^a\.example\.com$
RewriteCond %{REQUEST_URI} !^/a/
RewriteRule (.*) /a/$1

but it doesn't work. I use VestaCP control panel and don't want to add site for each subdomain instead have it all under the same root directory.

I also noticed that going to a 404 page like http://ift.tt/1Ohyu7q will show me the 404 page of the root wordpress installation and not /a/ installation

How can it be fixed inside .htaccess ? /a/ and /b/ are both multi site installation each and / is normal installation

the url for the /a/ and /b/ are set to a.example and b.example in wordpress settings