Do Perdo Vim spells still need to be aligned with a specific realm (as per the general guidelines in AM4?) That key sentence was dropped in 5th edition.
AM 4 PeVi guidelines, pg 158:
General: Reduce a Target's Might by one point for every point by which the level of the Vim spell + quality die exceeds it's Might. Different spells are required for creatures tied to different realms.
AM 5 PeVi guildelines, pg 160
General: Reduce a Target's Might by the level of the spell +10, as long as the spell penetrates the creature's resistance.
The main question here is:
If a character knows "Demon's eternal oblivion," can that character use it against all creatures with Might? Or does this only affect creatures with Infernal might. (In 4th ed., a similar spell would have to be learned to affect creatures with Faerie Might or Magic Might.)
It's not so cut-and-dry as that in 5th. For example, The Heathen Witch Reborn and Quiet the Cursing Tongue work on "any" Supernatural Ability that does that, which seems to strongly imply these effects cross the realms, being specific types of magic in a different way. But then look at Piercing the Faerie Veil and the related versions. I would say generally you specify a realm, but other types of specificity may bypass this. Generalization may work, too. For example, Wind of Mundane Silence seems to work regardless of realm.
In the specific case of Demon's Eternal Oblivion, there are different versions for different realms.