To stop renewing your current subscription (on www.vi.app, this is known as "Turn Off Auto Renewal"):
Step 1: In your VI App iOS app, go to "Subscription"
Step 2: Select "Manage Subscription in iOS"
Step 3: Select "Cancel subscription"
If you wish, to stop subscription immediately and request for a refund, you may contact Apple Customer Support at https://support.apple.com/en-sg/HT204084