Currently, Vivado Synthesis treats the instantiated cores as black boxes. At present, Vivado Synthesis does not synthesize or optimize encrypted or non-encrypted synthesized netlist. Hence, XDC constraints or synthesis attribute will not have effect for synthesis on an imported core netlists. This is supposed to change around the 2013.3 tools release timeframe at which time the core netlists will be passed to the synthesizer for optimization via a switch option.
Also, Vivado Synthesis currently does not read the core netlists and modify the instantiated components by default. This is supposed to change around the 2013.3 tools release timeframe under the above mentioned switch option.
However, Vivado Synthesis does synthesize secure IP RTL, and constraints do affect synthesis results.