Guest OS cannot access host OS VPN

[zyg]

Host OS : Mac OS X Yosemite
Guest OS : Win 10
Virtualbox : 5.0.10
Network config : NAT

I login VPN in my host OS X to access resources in the other side of the VPN. Then I want to access those resource within my guest Win 10.

Problem I met: I can access internet in my guest Win 10, but I cannot access the resource in the other side of the host OS VPN.

I can ping the IP of the resource in the other side of host OS VPN but cannot resolve the domain name of them. I think the DNS for the guest Win 10 does not work when it need to resolve the domain name for the resource in host OS VPN.
The DNS for guest Win 10 is automatic assigned by DHCP, and it is set to the DNS address from internet service provider. This is the reason why the guest Win 10 could not resolve those domain names in host OS X VPN. But how to fix it? Pls help!
Thanks!