Hello @Sab_goncalves that is true, a Fitbit tracker can not be linked to third party apps.
Third party apps like Vitality get linked to your Fitbit Account, not the Fitbit Blaze.
This linking would be done through the vitality website or possibly their app.
I suggest checking Vitality help site or their FAQ.
A quick search for me this YouTube video as a hit.
I'm moving your none hardware post to the third party app board.