In theory, your setup should work. As long as you have a line and a circuit between the two DECnet nodes, creating the DECnet tunnel is just a matter of defining the "tunnel" port, giving it a name, taking care of routing costs, turning on the system, and (most important) specifying a partner (the DECnet node name of the other VAX).
In practice, you may encounter these problems:
* With AppleTalk 3.0 for VMS, we don't recommend any tunneling below 56KB. In many X.25 networks, the connection between the computer and X.25 pad may be slower than the connection between the pads.
* The X.25 link should be up all the time. There is no way for DECnet PSI software to be dynamically connected and "know" when AppleTalk packets should go through. Even if the software could do this, the tunnel port definitions on both ends can't be dynamic.
* If your X.25 network has any two-minute delays, AppleShare will have problems. The ASP session timer expires at two minutes, terminating the session and losing the server connection.
* Another problem has to do with NBP lookup. You might not see the file server when opening the Chooser and selecting the AppleShare icon. Some sites have to change a resource called GNRL to increase the Chooser interval timer and retry timer values.
You can find a complete discussion of this subject in the Tech Info library in AppleLink. For more information, search on "AppleTalk and Wide and Area".