Vigilance is a canine companion that can be found most of his days at the Markarth Stables. Once there, the Dragonborn will encounter a man by the name of Banning, who offers the option to buy the dog for 500 gold. It is also possible to ask a few questions about Vigilance. In Hearthfire, it is possible to make Vigilance a pet.
When talking to Banning, he mentions that he and his family have been training dogs for the Jarl (and for his own use). He also refers to the dog as a War-Dog, and rightfully so. He is from a litter of dogs that were bred for fighting and protecting, which is probably why he is known as a vigilant war-dog.
This dog is fast and agile, meaning he can track down enemies rather easily. However he inflicts very low damage on enemies. He also has the tendency to dart towards an enemy, and can be utilized as a meat-shield, if need be. Although, this can be annoying for melee or ranged specific players, as the dog may often get hit. Note that Vigilance has absolutely no armor rating and will go down rather fast to physical attacks.
Like every other companion, Vigilance changes stance when the Dragonborn draws his or her hands, weapons, or spells. Instead of drawing a weapon like other followers, though, Vigilance starts to snarl and lower his body, creating an intimidating look.
Vigilance will level with the Dragonborn up to level 50 where he will have 500 hp, twice as much as Meeko and other Stray Dogs. Unfortunately due to a severe bug, his health is locked when first met, even after using disable-enable commands. One workaround is to force his health to 500 by using the console and typing "setav health 500" or simply waiting until level 50 before entering the map cell outside the Markarth gates where he can be found.
Vigilance is technically a mercenary, as must be paid the 500 fee each time he is requested to rejoin the Dragonborn.
Vigilance has the highest level cap of the dog followers.
If commanded to wait and then left for too long, he will return to Banning.
Vigilance cannot be a companion at the same time as another Dog.
Like all non-humanoid followers, Vigilance does not understand the concept of "stealth" and will charge straight at an enemy as soon as he sees one, even if the Dragonborn sneaks and hasn't been noticed yet.
He counts as an animal follower.
After downloading Hearthfire, he can become a pet and stay at your home. This is done after first, having bought/built a home, and then have adopted at least one child. By bringing Vigilance into your home, your child will ask if he or she can keep him, allowing you to keep Vigilance at your house and take him with you at your convenience.
It is more than likely that he has been fed human meat since his owner Banning, is part of the Coven of Namira a cannibalistic cult. This is discovered after completing The Taste of Death when the dialog option "Wait I know you?" is activated he will reply that now the Dragonborn knows the the secret ingredient to his spiced meat.
If the dog can't be bought again from Banning, his body must be found and the console opened (~); click on his body-(it must show the code) then write in console (resurrect). If his body cannot be found, Vigilance can also be resurrected by entering "Resurrect (0009a7ab)". He will be alive again but he will not be a follower. To let him be a follower again, go to Markarth Stables and speak to Banning and buy the dog again. It is also possible to try using setessential 0009a7aa making him just stand, when health runs out. When the battle is over, he will resurrect with low health.
If Banning has been killed during the quest The Taste of Death then there is no possibility to buy the dog. His help, Cedran, will only sell the horse but not the dog.
It has yet to be tested if resurrecting Banning via console will solve the problem. Fact is that his corpse vanishes when left rotting too long in Reachcliff Cave.
Confirmed (PC) method to obtain Vigilance after Banning's demise in Reachcliff Cave: [1] After wiping out the cannibals in the cave (if you choose to do so) and thus saving the priest's life, open the console and click on Banning so that his REF # displays. [2] Use the "Resurrect" command. ( Note: although the command will be saved, it will appear that nothing has happened) [3] Return to the interior of Markarth Stables and use the "player.placeatme 0009A7A8" command to place Banning near the character. [4] Purchase/ Repurchase Vigilance.
If placing Banning near the Dragonborn while outside the stables, Banning may return to fighting as if still in the previous cave (including his spectral wolf with him) causing the guards to take them both out, meaning that the process to bring him back to the Dragonborn will have to be repeated. Be sure to place Banning close while inside the stable house, just to be safe.
There also seems to be a glitch where if Vigilance waits too long and when returning to retrieve him it is possible to buy him but he won't remain a follower. He can't even be found. But when bought off of Banning again, he will say that there is already an animal follower.
PS3 In Hearthfire, if an animal follower (like Vigilance) is made into a house pet, your family and others within your home may only respond to your pet as "Stupid Dog", and very frequently. Sometimes this becomes one of the only interactions of dialogue heard throughout your time in your home. Normally after each time the dog barks, which is quite often.