cignus_pfaccari has it right.
You still get slayer advancement. Just, if you look at the progression in Heroes of the Forgotten Lands for a slayer, you'll notice at level 11, 12, and 16 there are several features that say are for 'paragon path' features (in the chart they are identified as 'mythic slayer features' and in the description detail, it says mythic slayer paragon path feature. Those mythic slayer paragon path features get replaced with the features from whatever paragon path you choose. (typically, we're talking an action point benefit at 11th level, a feature at 11th level, an encounter power at 11th level, a utility power at12th level, and a daily power at 16th level)
the other slayer features at level 13, 15, etc that aren't labeled as 'mythic slayer paragon path features' still accrue.