[RPG] How does a Vampire Spawn become a Vampire

I think the most important thing to consider is just how "mobile" mobile is. Sure you can pull a Vandread and lug a coffin with you, but then you have a coffin that can surely be whittled away it in less than an hour to get to your defenseless body. So while a coffin on the move is possible due to the unstated RAW, where it is when you go back to it is vital. Just how many places can you trust to see your wispy form go flying by and not capitalize on the opportunityto kill or imprison you? So while mechanically this sounds like a great advantage (I'll move my coffin every four days between seven castles), who can you trust with your life to do this?

Additionally, almost every D&D campaign I've been in has taken some liberties on the more classical lore. Your DM (assuming you're not it) might have different ideas as to just what a vampire's weaknesses and abilities should be.

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).