1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) Once you transfer a call, you no longer own that call and you are not connected to it in any way. So you can't "Grab" the call.
4) What you are trying to do is use a Vicidial function outside of Vicidial without any of the necessary programming or support of Vicidial. Instead, you should try passing the call to a logged in Vicidial agent in the Vicidial Agent Screen. Then you'd have that Vicidial parked call/grab call function. The Vicidial Managers Manual shows step-by-step instructions for setting up inbound. Free Version includes it, too.
5) Since you're operating outside Vicidial ... you could try an Asterisk MOH method instead, which probably would work.
You have a Limo and you're having the clients ride in the trunk ... while wondering why they are uncomfortable. LOL
(Sorry, had to!).