we have exactly the same issue. 2 out of 3 vivo fit jnr devices work seamlessly. 1 of them says it's linked to another account. Option is to reset it but upon selecting reset a message appears saying to 'verify your connection and try again'. We've tried removing the battery, pressing and holding the button etc etc. Deleting the app and re installing, deleting bluetooth pairing and re-pairing. Nothing!!