[RPG] What happens when Resurrection is cast on a zombie

Raise Dead — No. The spell description, at the end of the second paragraph, states:

The spell can't return an undead creature to life. (PHB, p. 270)

Revivify — Probably Not. "[A] creature that has died within the last minute ... returns to life with 1 hit point." (PHB, p. 272) Both Animate Dead and Create Undead have a casting time of 1 minute, so their targets could not become undead before Revivify is ineffective. Similarly, many undead monsters take well over a minute to make (e.g., liches, mummies, vampires). However, there appear to be other ways to create undead in less than a minute, for example, the Finger of Death spell and death tyrant's Negative Energy Cone raise zombies the next turn, a shadow dragon's Shadow Breath creates a shadow the same turn it kills a humanoid, and the wraith has the Create Specter ability. These cases may require DM adjudication. One way could be by Contest (DMG, p. 238; PHB, p. 174) for the soul to take over its undead body due to the opposing magics involved. See generally, Magic Jar spell description (PHB, p. 257). But see, Discussion below.

True Resurrection — Maybe. The spell states, "If the [dead] creature's soul is free and willing, the creature is restored to life ... the spell can even provide a body if the original no longer exists." (PHB, p. 284) This suggests that incorporeal undead are resurrected if their soul is free, as the spell would simply create a new body. For corporeal undead, the question is whether True Resurrection can supplant the undead force with a free soul. (If permitted, this could be done by Contest. See above.)

Ultimately, the DM must decide whether being undead somehow makes a soul not free and thus unable to return to life. See Discussion below.

Discussion

Although not explicit in its description, Revivify probably does not work on an undead creature without a body given that it requires "touch[ing] a creature that has died within the last minute." (PHB, p. 272) According to the Monster Manual, a specter is a "bodiless spirit" (p. 7) and has Incorporeal Movement (p. 279) so True Resurrection may work by creating a new body. However, the specter's description suggests that it is the soul itself—"Some are spawned when ... the touch of a wraith rips the soul from a living body" and "its only end the oblivion that comes with the destruction of its soul" (Ibid.)—implying that the soul is not free to be restored to life.

If a creature is killed by the spell Finger of Death, "it rises at the start of [the caster's] next turn as a zombie." (PHB, p. 241) According to the MM, "Once turned into a zombie, a creature can't be restored to life except by powerful magic, such as a resurrection spell." (MM, p. 315) Reading way too much into this, "resurrection" here is not italicized so it is not the specific Resurrection spell. However, it does say "powerful magic" and a 3rd-level spell like Revivify is not powerful magic, so I don't think it would work; but, True Resurrection is the most powerful resurrection spell and might depending on your DM's interpretation.

They can, with a catch and a few assumptions.

The only way RAW I could find to resurrect the victims using the Resurrection spell is by first casting Greater Restoration in order to remove the effect of the vampire's bite (since the victims are not vampires yet), and then cast Resurrection. The Resurrection spell on its own would fail, since the bite is still in effect. This would need to be done within 10 days, as otherwise the effects of Gentle Repose would go away and the victims would rise as free-willed vampire spawn.

This however, assumes that the vampire's bite, or at least it's health reduction component is a magical curse, that Greater Restoration can be cast on a corpse that is soon to rise as an undead without intervention (on the same principles that sleeping creatures are nevertheless creatures, even though the victims in the case would be temporary corpses if left unattended), and that Greater Restoration covers vampirism (assumption made from lycanthropy being able to be cured by Greater Restoration).