network related or instance specific error occurred while establishing a connection to sql server

Hi all,

I am getting  a SQL error - A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified).

Problem is when a user tries to connect with remote SQL Serverthrough VPN from Domain, it throws this error, and when the same machine tries to connect with Remote SQL Server through VPN as a standlone machine, it is getting connected.I checked all the settings but ....... There is no any policies deployed in Active Directory.

Any suggetion?

Rajneesh