Heimdall: Fenrir cannot be defeated, at least not by any of us. Vidar was destined to kill him, only after Fenrir killed Odin.
On the last day, when all the forces of good and evil will meet for a final battle. Fenrir will seek out Odin, and Odin will seek out Fenrir. The two will battle each other for a long time. Eventually Fenrir will overcome the Allfather and kill him. At that moment Vidar will step forward and grip Fenrir's jaw. Placing one foot on the wolf's lower jaw and gripping his upper jaw with his hand Vidar will rip Fenrir asunder.
Idun: When Fenrir kills Odin, something happens to Vidar to increase his already immense strength. Also special boots and gloves will protect Vidar from the teeth as he grips Fenrir's jaw. However, since Odin was not killed by Fenrir and since he was killed to soon Vidar cannot defeat the wolf.
Heimdall: This is pointless since we are not here to defeat Fenrir. We are here to stop the holocaust and prevant him from being released by finding the Time Matrix. Come the room where he was kept is only a little farther away.
*Heimdall and Idun leave the room and head to Fenrir's holding chamber.*
"Dulce bellum inexpertis."
Official Forum Expert on Norse Mythology
As Odin says in the Hovamal:
"Praise no day 'til evening; no wife 'til on her pyre; no sword 'til tested;
no maid 'til bedded; no ice 'til crossed;
no ale 'til drunk."