My hope laptop is connected to the VPN and I have added this machine to the domain. After this when i try to logon to the domain, it give me follwoing error:
"Logon server is not available error "
Any idea why this can be happening?
Just a guess here, but you might not be able to connect to a VPN until after you log on (what OS are you using?). Meaning if you connect to the VPN, then join the domain, then log off, you are now on the domain but off the VPN so you will definitely not be able to log on.
Again this is just speculation.
Most likely DNS isn't resolving after you VPN in. One thing you can try is connecting to the VPN, and then trying to use nslookup to check whether you can resolve the domain.
If it can't resolve the server, you may need to have your VPN administrator verify the DNS settings for VPN clients.
This depends on what type of VPN you are using. If you're using a hardwire vpn (box to box), like MarkF said, you're probably having a DNS issue. You might add your domain to your hosts file.
If you're using a software VPN, like Puddingfox said, you'll need to login first before you will be able to connect to it.
In either situation, you should still be able to login using cached credentials if you have ever logged into that domain with the user name you are trying to use before.