When a max_fanout attribute is applied to a register which has a constant VCC/GND input, the attribute is not honored and the following message is issued:
This is a known issue that is fixed in Vivado 2015.3. The problem occurs due to the lack of proper constant evaluation of the inputs. Vivado Synthesis fails to honor correct replication when the input to the flop is a constant.
A possible work-around is to create a dummy register, as in the following example: