I'd like to create a list to keep track of all the features we could make use of with VIP level and Campaign stages. Feel free to create a new comment per feature, so we can discuss features inside each comment.

Every entry here means that the player has unlocked the specified feature if they are on that level. So for example, the Oak Inn gets unlocked after completing chapter 4. This means, that if the player is still on the last stage of chapter 4 (4-36), they have not yet unlocked the Oak Inn. Only after beating chapter 4 and entering chapter 5 (5-1) has the Oak Inn been unlocked. With this in mind, the stages/chapters/VIPs are always equivalent to the first entry they are capable of that feature. In this case, Oak Inn is unlockable only with 5-1 and not 4-36.

Ranhorn

• Store:
  • Store: 2-9
    • PoE coins?
    • Elemental Shard: 22-1
    • Elemental Core: 24-1
  • Guild Store: 2-21
  • Labyrinth Store: 2-1
  • Challenger Store: 9-21
• The noble Tavern: 2-1
  • Stargazing room: VIP 13 or 16-1
  • Temple of Time: VIP 13 or 16-1
• Oak Inn:
  • Friend Inns: 5-1
  • Create own Inn: 17-1
• Guild: 2-21
  • Twisted Realm: 13-1

Dark Forest

• Arena of Heroes: 3-1
  • Legends' Challenger Tournament: 6-1
• King's Tower: 2-13
  • Four original faction towers: 15-1
  • Celestial and Hypogean towers: 30-1

General

• Mercenaries: 7-1
• Quick-battle Guild: VIP 6
• Skip battle in the arena: VIP 6
• Bounty Autofill and Dispatch: VIP 6
• Auto-fill Heroes in quests: VIP 6 or higher (or stage 12-40) (????)
• Fast rewards:
  • 16-1 - Buy once
  • 24-1 - Buy twice
  • 32-1 - Buy three times