I have had no problems with separate php_error logs per vhosts in the past.
I have recently installed a server with Ubuntu 16.04, using php7 on apache2.
my vhost reads:
<VirtualHost *:80> ServerName examp.com Header always unset X-Frame-Options DocumentRoot /var/www/sos/rtg/html ErrorLog /var/www/sos/rtg/error.log CustomLog /var/www/sos/rtg/access.log combined php_flag log_errors on php_flag display_errors on php_value error_reporting 2147483647 php_value error_log /var/www/sos/rtg/php_error.log php_admin_value error_log "/var/www/sos/rtg/php_error.log" </VirtualHost>
Based on internet recommendations, I have touch 'ed php_error.log and adjusted privileges. chown and chmod all different possibilities. I cannot seem to get php to write to an error log other than apache's error.log
I have had this working in all other servers I have (albeit ubuntu 14, apache2 and php5)