I like Vilefiend but I assume it was only taken when we used Netherportal for the extra 1 shard summons and since we do not use NP with Explosive Potential trait, it brings Soul Strike to the top single target dps?
I think this is fight dependent. I haven just started getting into demo (I've had shit luck with azerite traits) and I've played with both. If you're able to stand still and cast heavy (Champs,Grong,etc) then Vilefiend feels fine to me, but its damage is very similar on those pulls with SS, which is very helpful on high movement fights (Opulence,Jaina,Conclave,etc). SS is also nice if you need a fast switch for w/e reason as it's a built in pet attack ability.
For M+ I've been playing Demo for awhile, and SS has always felt nicer. M+ generally have a lot of movement and having the extra shard on a short CD feels great for IMP or BB. So in this area of the game I'd say its the only choice.