As effective as CO2 is against most vicious dogs, there are a few rare ones that are unaffected by it - and they will continue to attack. For those rare few, deadly force will most likely be the only remaining option. These dogs tend to be either "trained" to attack or severely abused and impervious to pain. The use of deadly force in these circumstances is readily justifiable, especially if you've attempted a less-lethal option (CO2) first.
Before I continue, I have a recommendation on personnel assignments. Team leaders need to KNOW their personnel - their strengths, weaknesses, etc. When it comes to assignments involving known vicious dogs, it's imperative to assign the right officers to the front: those with ice in their veins who are not afraid of dogs and will wait until the last second before pulling the trigger on a charging animal.
Conversely, there are officers who are legitimately fearful of dogs (beyond the expected massive adrenaline overload). They may be outstanding, courageous officers in every other way, except when it comes to dogs.
You don't want to put the "fearful" officer anywhere near the front of the entry team. Unless you want the dog shot and possibly killed from 100 feet away. As with everything else in SWAT, certain officers are better suited for certain assignments than others (due to size, training, temperament, etc.). This is doubly true when it comes to dogs.
Pick the right person for the job, and the potential for "Fluffy" to be shot is drastically reduced. Pick the wrong officer, and you'll likely regret it.
A word about Animal Control. Although trained, equipped, and effective at controlling dangerous animals of all kinds (including vicious dogs), Animal Control is not SWAT. My opinion is it's unsafe, even for armed Animal Control, to enter any unsecured premises.
Once the premises have been secured, by all means, Animal Control is a welcome presence in removing dangerous animals. However, to put Animal Control in harm's way is not something I'd recommend, for the simple reason that they're unfamiliar with our tactics because they aren't SWAT.
When it comes to shooting vicious dogs, the circumstances are anything but optimum. The dog will usually be charging, growling, snarling, biting - a fast-moving, swirling, low-slung, extremely difficult target to hit. The surest shot is the close-up shot from near contact distance.
Team leaders are advised to designate a "dog shooter" for when less-lethal means aren't effective or available and lethal force is needed. Again, this person should have the temperament to shoot from up close to ensure zero misses. Having a designated shooter allows the rest of the team to remain focused on the real target, instead of the distraction - the dog(s).
Our team learned this valuable lesson in what became known as the "Dogman" incident. It was a drug raid on a residence after dark - nothing unusual, including dogs, fortifications, etc.
The point man found the front door unlocked, and after announcing "PO-lice!" he entered what turned out to be a darkened enclosed porch. He sensed "movement" to his left - a charging Pit Bull that latched his powerful jaws directly onto the point man's a**. Out he came - spinning to get the Pit Bull off.
The entry team tried to get a bead on the spinning duo; however, no one could shoot for fear of hitting the point man. After a number of "spins," the Pit Bull fell away and took off at a high rate of speed. The raid proceeded without further incident, followed by "Dogman" getting stitched up at the ER.
The debrief SWAT sergeant made this startling observation. While "Dogman and friend" were spinning in the front yard, the entire entry team was focused on them. No one was covering the target itself, leaving the team vulnerable to the real danger: getting shot by the suspect.
After "Dogman," our team began to designate "dog shooters" combined with CO2 against vicious dogs. This system has worked extremely well for many years, keeping SWAT dog shootings down to a mere handful. Which is a lot better than 100-plus dog shootings a year, a situation which place your team under the microscope unnecessarily.
Ultimately, you have the right and duty to protect and defend yourselves and your team, using the means you deem necessary. While I know what's worked well for our team, what you choose is entirely up to you.