To activate inbound on a system already using outbound, the first step is to get the call to arrive at the dialer. There are two basic ways to accomplish the arrival:
1) Modify a setting in your carrier's web page to "point" the DID to the IP address of your server.
2) Register your vicidial server to an "account" on your carrier's system that already has a DID assigned to it. (Which may also require step one above to "assign" the DID to that "account".)
To validate that the call is arriving at your server, you will need to use the asterisk command line in sip debug mode. If you get ANY activity when you dial the DID, you have succeeded in step one. If you do not get any activity, try to turn off your firewall and test again (avoid doing this during operating hours/times as firewall changes can be disruptive). You can test and turn the firewall back on in a couple minutes total, so a break or lunch are good if you must do this during operating hours.
The next stage involves turning control of the call over to Vicidial (not just asterisk, but to get the Vicidial scripts to control the call so the Web Pages for Vicidial will determine the call route because those web pages write data that the Vicidial scripts read). This is actually quite simple: "context=trunkinbound".
The "context=trunkinbound" should be applied to whatever Admin->Carrier "Account Entry" is used to link to the carrier. If the inbound call is coming from the same address as the outbound calls are sent, then put it in the existing outbound carrier. If not, then create a new [context] (and if you like, a whole new admin->Carrier!) and be sure the "context=trunkinbound" entry is present in the account entry. If you create a whole new carrier for this, leave the "Dialplan Entry" area blank. Inbound carriers can have a "registration string", but it is usually only necessary to have one IF the carrier is not using "IP authentication".
Once this has been done successfully, an inbound call should be answered with "This number is not in sevice, Goodbye!" as that is the default behavior (default DID under Inbound->Show DIDs). This recording playing to the caller, and the associated asterisk command line showing this recording playing, indicates that you have succeeded in configuring your system to receive inbound calls.
Now you read the Vicidial Manager's Manual to figure out how to get it to an agent!