Mooneyboy
Your concern is right. I think we were misled by the assumption that with Slat extended delta vref is Vref+10. There is no reason why would it ignore other effects of dual Hyd fail. With slat extended all that changes is you do not have to lower gear at 200kts. but rest of the STATUS remains applicable which says:
APPR SPD..................................................VREF +25 KT
Approach speed must be increased, due to the loss of
ailerons and slats.
LDG DIST PROC...................................................APPLY
and landing distance should be found from Dual Hyd Fail G+B and not from SLAT/FLAP procedure.