Windows Server on different vlan with different firewall

Hello, I was in the Windows Server forum and they directed me to go here.

Our company has decided to deploy a RDS solution for our 20 offices. The files and such the branch offices will need access to are not the same as the ones for corporate. So with that said I would like to put the RDS farm on it's own vlan. Currently our domain controllers sit on vlan 2 and vlan 2 is routed by our firewall. 

My plan is to put the RDS servers together on vlan 25 and add a second firewall for the traffic to flow through for RDS. This second firewall would also do the site to site vpn needed for the users to connect via rds since we aren't doing it through a web portal. I would like to make this one big AD and not split it up so we can manage it easier.

My problem lies in how do I get the RDS vlan to talk to the domain controllers to be part of the domain and still make the RDS vlan go out the second firewall.

My only solution has been to setup RDS on its own site and then do a site to site vpn between the two firewalls and that way I can have access to vlan 2 where the DC's are but would be able to block access to other vlans.

I've copied the link to my original forum post.

https://social.technet.microsoft.com/Forums/windowsserver/en-US/6a495a1e-93c3-4231-a8bb-86841be49dad/new-child-domain-question?forum=ws2016#826a3bc6-3bf4-4e3e-8453-13acaa345ef1

Thanks for the input.