If certain levels of activity are not logged every month customers are charged extra for the device.
The Apple Watch usually costs £369 in the UK.
The firm said a small number of customers were affected and some had synced their devices incorrectly.
Vitality declined to give figures of how many had taken up the offer.
Under the deal, customers who do not submit enough activity points per month have to pay an extra sum of up to £12.50 every time they fall behind.
The contract lasts for two years.