I have 16 threads.don't bother with piping tar to pigz, do it separately for more reliability and speed
so tar first and then pigz the .tar file
FYI, pigz will only speed up compression compared to gzip if you have more than 1 cpu thread available
cd /var/www/vhosts/ && tar -cfp httpdocs-`date -I`.tar httpdocs/ && pigz --best httpdocs-`date -I`.tar && mv httpdocs-`date -I`.tar.gz /backup/files/