Some things should never change when fielding Vets, regardless of the edition.
Special Weapons choices should always be Melta or Plasma....all the rest are lesser choices by either been poor at actually doing much damage (Snipers/GL's), or not utilising the BS value (Flamers).
Some IG'ers crap on about Get's hot been a killer for using Plasma, so forgo using one of the most powerful and effective weapons for lesser ones....what a hoot, under - utilising our best Troops due to a 1 in 6 chance of dying, unfreaking believable.
Personally though, I would consider suicidal 5-man ST Squads with 2 Meltas to do the early game backfield AV duty, which makes it easier to accept that ChiMeltaVets hold back for those first few turns like I say further down.
While I'm on the subject of weapons, the most useful and versatile Heavy Weapons are AC and Lascannon, the rest are poorer choices unless as used as Hull weapons on vehicles (HB/HF).
Vets never ever should be on foot, well until forced out of a vehicle...T3 guys die far too easily to far too many widely used enemy weapons.
I disliked Vets in Vendettas in 5th Ed but seen as the Flyer is now more durable and likely to last until late game turns I can see having Plasma Vets for late-game Objective taking.
Vets in Chimeras are still viable but need to be held back for a few turns so HP's don't get stripped off too early, and leave the Vets on foot and in range of too much enemy firepower once that happens.
Mech lists should have the maximum amount of vehicles possible, so for that reason I don't see Hybrid foot/mech lists been that effective now. So Vets in Chimeras, Artillery at the back in an ADL, and Hellhound and Russ varients out front moving forward slowly.
Doctrines have never been popular, especially not on multiple Squads, because it's far more effective to just field more bodies.
Bear in mind I'm talking about competitive gaming here...this is the Tactics Forum afterall.