The healthy food amount is never as much as the monthly fee for Vitality. Most of the offers are pointless. Basically spending money on things we wouldn't buy otherwise to get a discount.
Our groceries remained fairly static - but the payments were erratic, some months nothing, some a R100 - some R700. One of my biggest gripes is you have no way of verfiying what you get back (e.g. Food/Dischen/etc).
We probably made R200-R300pm more than the fee while on Diamond with Vitality.