Ah vehicle squadrons. I see a lot of hate for these guys and a lot of comments like "lol why would you take russes in squadrons?" relating to how vehicles can be easier to kill when in squadrons due to dying on immobilised results. Whilst this is certainly true, there are a lot of other factors which are not considered. Let's take a look at vehicle squadrons in general so we can get a better understanding of they work in actuality.
The most obvious bonus of vehicle squadrons is you can fit more into your Force Organisation chart. In 5th edition where everything is cheaper and armies have inevitably become bigger and more balanced, this is huge for some tanks which didn't have this option before (Imperial Guard codex jumps to mind as taking advantage of this most). You obviously lose some fire potential however as you can no longer spread that firepower out over multiple targets which can often lead to massive overkill (hi Imperial Guard again). At the same time you make your own vehicles 'more vulnerable' as a 4+ (or 50%) on the vehicle damage chart kills them (compared to a 5+ normally). With potential bonuses to this such as open-topped status or AP1, vehicles in a squadron can be very vulnerable indeed (how easy do Piranhas die in a squadron against meltaguns when they go flat out for example?). The flip-side of this is the squadron can always move (unless it is destroyed) as stunned results turn into shaken results and with immobilisation results being upgraded to wrecked, if the squadron retains squadron status, it is always mobile. Not super significant but important and often over-looked.
This doesn't encompass the whole picture though. In the basic scheme of things, the opportunity cost of essentially extra armor for an increased chance of dying is too steep. Luckily we like to look beyond the basic scheme of things. To take advantage of a vehicle's greater chance of dying in a squadron you have to cause multiple penetrating hits at least equal to the number of vehicles in the squadron (and preferably two per vehicle to be 'statistically' correct). Why? Because hits get assigned to individual vehicles much like 'wound allocation shenanigans.' This means unless you saturate the squadron with penetrating/glancing hits, hits will continually be assigned to a single vehicle. This can raise the survivability of specific vehicles within a squadron dramatically (i.e. different Russ types in a single squadron) and allows the squadron to maintain offensive effectiveness by making it harder to suppress the whole squad. When used correctly this can counter the increased effectiveness weapons have against vehicle squadrons.
How then does this apply against different armor values? Obviously there's a huge difference between a Land Speeder squadron and a Leman Russ squadron. Against lighter vehicles, saturating the squad with rolls on the damage table isn't that hard. A Predator w/heavy bolters and autocannon for example is on average, going to generate ~2.2 rolls on the damage table against AV10. Not too hard to generate enough rolls on the damage table at one time then to spread the hits out on the squadron. On the other hand, doing this against AV14 Leman Russes is a lot harder. Some squads like Fire Dragons (multiple meltaguns) will just laugh at this concept but those squads are few and far between. Being able to therefore get multiple rolls on the damage table against AV14 can be very difficult which can lead to the benefits discussed above.
Now this is obviously an expensive proposition. The ability to have a Leman Russ shooting more often than not sounds great but having to buy another Russ for this to happen...not always feasible and when they are both shooting...well whatever you're shooting at is likely to become vaporised. Is this price effective? Not really. It's viable certainly but is cost prohibitive and therefore unlikely to be seen or used regularly. Cheaper vehicles like Land Speeders and Piranhas or more expensive vehicles like Vendettas/Valkyries are obviously more realistic options. Although these types of vehicles don't take advantage of vehicle squadrons as much as higher AV vehicles such as Russes, they still gain benefits to counter-act their reduced durability as discussed above. If an opponent doesn't generate enough hits to put damage rolls on every squadron member or cover stops some of them, the squadron can still be very potent offensively.
Which brings us to our final point about vehicle squadrons. Cover. Generating cover for vehicles is more difficult than for infantry models as an actual 50% of the model has to be covered. With some large vehicles or skimmers on stands this can be particularly difficult or at least more difficult than with infantry. We've covered generating your own cover on vehicles before so we'll use that as a premise. To generate cover for a vehicle squadron, at least 50% of the vehicles in the squadron must have cover. This is means only one vehicle in a two vehicle squadron has to have 50% of the particular armor facing covered which is a lot easier than 50% on every vehicle. This makes generating cover for vehicle squadrons a lot easier than normal vehicles and can allow vehicles to operate in the open and get clear LoS on opponents whilst having cover in return (Tau players, unite!). Remember as well there is a 4" coherency rule for vehicle squadrons which makes spreading them out to take advantage of this much easier.
When we combine this understanding of cover with the previously discussed understanding of vehicle squadron durability, we can attempt to fob off as many hits as possible onto particular vehicles in a squadron and then ensure that vehicle has cover. This allows the other vehicle(s) in the squadron to operate at peak efficiency whilst having a somewhat improved durability and easier access to cover.
Summary
Vehicles in squadrons are certainly vulnerable to multiple penetrating hits at the same time, particularly from melta weapons. The most basic of trade-offs for this reduced durability is always being able to move but when we look deeper we can see there are further benefits. Easier access to cover is the most readily applicable but if there are not enough damage rolls are generated against a squadron, the durability of a specific vehicle in the squadron can sky-rocket and be very hard to suppress. This is a lot easier with higher AV vehicles but very cost prohibitive. That being said, it is still very important for lower AV squadrons such as Land Speeders, Piranhas, etc. and makes these vehicles a lot more durable over a whole game compared to a single vehicle counterpart.
Beyond all this discussion on survivability, durability and efficiency, vehicle squadrons allow you to fit more into a single Force Organisation Chart. What this provides armies is more firepower in a localised area and allows an army list to pack more into less (dependent on points obviously).
In the end vehicle squadrons are an important part of scaling for certain lists (Space Marines!) and offer up quite a few tactical and army building options for players.
