Although your vendor A has the fixed source of supply but its validity till 30.06.2014.
It means the source of supply will be valid only till 30.06.2014.
But system is trying to fetch the source of supply as on date 09.07.2014. So source of supply for Vendor A will be not valid for this. So vendor B is picked.
Actually fixed vendor will work in that case, if you have more than one source of supply within same period, but you have the fixed source of supply for one vendor, then it will pick the fixed source of supply.
PR delivery date will be applicable to find the source of supply.