Configure a DNS server in ISPConfig 3

the ispconfig dns server is serving zones with public ip's configured in them?
is the other vm in your local network reachable via a public ip?
does the proxy target vm host multiple sites?

if the other local vm has public ip's, then just create the sites on that vm with different subdomains and give them the public ip of that vm in dns

otherwise as far as i can see, you can have each site on it's own local ip, and just redirect using the internal ip's.
or you can have each internal site use a fake name, eg site1.internalvm.com, site2.internalvm.com and have those configured in the hosts file on the proxying server and the internal vm, and configure proxying to those fake names.

it may be possible to do something with split dns, but that may be over complicating things, and i don't believe ispconfig currently supports the use of split dns.