We sync data from the Garmin Connect/FitBit accounts, not the devices themselves. Make sure your data from the wearable is in sync with their respective online accounts so that Vantage Fit can fetch it from their servers. 

To force a synchronisation, open your wearable app (Fitbit/Garmin/etc) and sync your wearable. Freshly open the vantage fit app and sync in the vantage fit app.