In fact, the more I look at your DNS set up, the more I think it's wrong.
You are pushing 192.168.1.1 as a DNS server AND 192.168.1.100. Which one is your correct DNS server? Delete the other one.
If the router is your DNS server, then you definitely need to add vtun0 as a listen interface, as I've said above.
If 192.168.1.100 is your DNS server, then you don't need to add vtun0 but you do need to delete the openvpn-option pushing 192.168.1.1 as the DNS server to clients. In this case, you also need to ensure 192.168.1.100 will respond to requests from the VPN subnet of 192.168.100.0/24.
If your correct DNS is the router,
configure delete interfaces openvpn vtun0 openvpn-option "--push dhcp-option DNS 192.168.1.1"
delete interfaces openvpn vtun0 server name-server 192.168.1.100 set interfaces openvpn vtun0 server name-server 192.168.1.1
set service dns forwarding listen-on vtun0 commit save
If your correct DNS is 192.168.1.100, also make sure 192.168.1.100 listens for requests from 192.168.100.0/24 and:
configure delete interfaces openvpn vtun0 openvpn-option "--push dhcp-option DNS 192.168.1.1" commit save