Two Postfix and one dovecot

Try setting up your dovecot LDA with the following parameters:

argv=/usr/libexec/dovecot/deliver -f ${sender}
           -d ${user}@${nexthop} -a ${recipient} -n -m ${extension}

I use this to support extensions in the local part (like test+xyz@example.com). You may want to try ${mailbox} in place of ${user}@${nexthop}. Have a look at man 8 pipe for all possible substitutions.

BTW: This is used by me for a working virtual mapping which maps email adresses for multiple domains into virtual mailboxes. On dovecot's side I'm using the same tables (via dovecot's SQL configuration) to do authentication for each virtual mail user. Not sure if you can map virtual mail addresses to real system users this way.

Yes, you can't have domain in both mydestination and virtual_mailbox_maps. To overcome this you need set mydestination in main.cf

mydestination = localhost.$mydomain, localhost

and in /etc/postfix/virtual

root@domain1.com   root@localhost
webmaster@domain1.com  webmaster@localhost

with this setup, email to root@domain1.com will be redirected root@localhost. Because localhost listed in mydestination then postfix will apply alias in /etc/aliases