Vanguard Veterans are an elite version of the standard Assault squad, composed of Veteran Space Marines with access to lots of specialist weaponry. Fortunately, they dropped massively in point cost from last edition, but their Heroic Intervention no longer allows them to assault when they Deep Strike. They now ignore the penalty for disordered charges and automatically pass tests for Glorious Intervention. These are handy but situational advantages; they won't change the game like assaulting from Deep Strike but the squad has decreased in cost enough that it's worth it.
You include Vanguard Veterans for their equipment, not their special rules. Any model can take melee weapons such as power weapons and power fists, as well as grav and plasma pistols and storm shields, and the Veteran Sergeant can take a relic blade. If you want a jump pack squad that can take out heavy targets, Vanguard Veterans are your best (really only) choice. Don’t go overboard on the melee weapons; three or four are enough to wipe out most enemy squads, and they get expensive quickly. Alternatively, I’ve heard that some players have had success arming every model with two grav-pistols. These can Deep Strike and produce a ton of very dangerous but short-ranged firepower. This seems very expensive to me (I would prefer to just take a Command squad with plasma guns and a Drop Pod) but it certainly packs a punch.
Vanguard Veterans are at their best when deployed with jump packs. If you want an assault unit on foot or in a vehicle, you’re better off with Honor Guard or Assault Terminators. They pair particularly well with Shadow Captain Shrike, as he allows them to Infiltrate.
If you’ve been reading my blog for a while, you know I love Sternguard Veterans. They all have Special Issue Ammunition, which makes them the best swiss army knife in the game. Dragonfire rounds ignore cover, while hellfire rounds wound on 2+. Kraken bolts are AP4 with 30” range (very useful if you’re 15” from the enemy and want to rapid fire). Vengeance rounds are risky because they get hot and only have 18” range, but they are AP3 so they’re useful if you really need to wipe out marines in a hurry.
They’ve got a lot of ranged options; two models can take a special or heavy weapon (including the rare heavy flamer), and any model can take a combi-weapon or storm bolter. I wouldn’t bother with many of these, as they all replace the extremely useful Special Issue Ammunition; combi-weapons alone retain this flexibility while adding some more specialized firepower. I prefer to use my Sternguard as close-ranged firepower deploying from a Rhino, so they often carry two heavy flamers. This limits their firepower at range but makes them extremely dangerous up close. Also, few squads want to charge a target with two overwatching heavy flamers.
Sternguard should be deployed in a Rhino or Drop Pod. They need to reach bolter range without taking too much punishment. Sternguard can pump out serious firepower but they die just like normal Marines. Despite my general disappointment with Pedro Kantor, he does give your Sternguard units Objective Secured and is worth it if you are taking multiple squads of Sternguard.