I can't tell if you added another Oracle client home for your vb app, or if you're using the existing 9i home.
The usual suspect here is multiple homes and TNSNames.ora are not shared across Oracle client homes. You need to either add the entry in each home, or set TNS_ADMIN environment variable to the folder you want clients to pick up the files from. Maybe TNS_ADMIN is already set on the problem machine, and therefore not using the tnsnames.ora you think it is?
Also, less likely, but names.default_domain in sqlnet.ora can muddy things up. If it's set to "world" for example, then your app would need to use alias.world to connect.