The step from Spawn to Vampire is described on page 295 of the Monster Manual as:

If a true vampire allows a spawn to draw blood from its own body, the spawn transforms into a true vampire no longer under its master's control.

Interestingly enough, that suggests that any true Vampire can promote a Spawn, not just the Vampire that created the Spawn.

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).