I am I right in assuming that it either of the Vcc rails fails then the outputs will go in high impedance states and the no damage will be effected on the chip?
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Thats what I summize as well from the data sheet, but what if one the power rails is effectively disconnected ie floating? - I a more concerned about potential chip damage
"Floating" is not a voltage. Assuming that you have proper decoupling cacapaitors, those capacitors can still contain a charge, i.e., the device is still powered. But this depends on how much supply current your entire circuit is using.
No damage is expected if one of the Vcc rails is unpowered assuming it still is within the absolute max ratings we set for the device. We do have a clause in the datasheet which states that the VCCA is not to exceed the VCCB so if you do remove VCCB and leave VCCA powered, you may see leakage current from VCCA pin into VCCB pin.