Land Speeder Vengeance
The Dark Angels are known for hogging several technological marvels for themselves and their unforgiven successors that for some reason never quite reached the Adeptus Mechanicus like most such discoveries are meant to....meaning the Angels are not good at sharing shit.
The Speeder itself
To briefly describe the Land Speeder Vengeance then, if a normal land speeder could be compared to an assault bike, the vengeance is a flying tank. It is a larger version of the Land Speeder with more dakka that serves as a quick response heavy weapons platform. It's speed can let it fly to a troublespot quickly and it's plasma storm battery can make melted smears of the enemy in equal amounts of time. The downside: it's a 140 point Land Speeder. 50 for the Speeder, 90 for the plasma, and no protection on your AV10 flying kill-machine beyond a basic jink. Deploy it wisely.
The Darkshroud Land Speeder Vengeance
Like any religious chapter the Dark Angels also like to kit their Vengeances out sometimes with a holy relic instead of a deadly super weapon instead. This is not just for show and tell however; the relic, the stone guardian, is a potent arcane creation capable of creating a barrier known as the shroud of angels which cloaks the vengeance in an aura of darkness that can repel even the worst of enemy fire. This does come at a two-fold cost, however. First off, the statues are soaked in Warp energy, and the stormclouds are caused by said Warp energy. Second, you're dropping the most powerful weapon on the vehicle, so it is a bit of a toss up between more protection for your army or more heavy firepower. Decisions, decisions....
Actually, it's usually a pretty easy one: are you running more Ravenwing than everything else? If yes, take the Darkshroud. You'll thank us later when your basic bikers get a 3+ jink save and your Black Knights get a 2+. Otherwise, feel free to splurge for that Vengeance and go to town on anyone bold and foolish enough to run TEQs against you.