RD Session host login when DC unavailable

Hi,

We're using a RD Session Host on a remote site, joined to the internal domain.
There's no DC on this remote site (connection to DC though a site 2 site VPN).
Connections to RD Session Host are made using domain users accounts.

My problem: if the VPN is down, RD connections are refused by the RD Session host:
"The system cannot log you on now because the domain <DOMAIN_NAME> is not available."

I would like the users to be able to login using previous users' logon information
locally cached on the RD Session host server.
How is it possible ?

Thanks in advance,
Vincent