I agree with @tianon , source /etc/apache2/envvars should be added to apache2-foreground script. I faced the same situation, I added a vhost which need to specify the log directory to {APACHE_LOG_DIR}/xxxx-access.log, however, without the source statement, I got following error:

AH00111: Config variable ${APACHE_LOG_DIR} is not defined

I have to create a copy of apache2-foreground with the source modification to make it boot.