I brought this up on shadowpriest a few days ago. The trinket bumps up the value of crit slightly. Essentially what Twintop just said - it goes from being the lowest stat to being about equal to mastery.
Edit: Additionally, on the subject of stat weights, they scale with gear, so mastery can be anywhere between .4 and .5 depending on the level of gear the sim is run under. What matters the most is order in which the stats fall - ie whether crit is lower or higher than mastery, etc.. A difference between my stat weights and twintop's probably just signififies that we ran simulationcraft under a different set of gear. I would be alarmed if the order of the weights changed between two gear lists, but otherwise the gear should basically come out to be the same order in the lists.
Edit3: Quick glance at Twin's stat weights - basically the only difference here is that I've driven down the value of hit to match mastery, even though it is worth more, because I intend to only get hit from reforging mastery or crit and never haste.
Edit2: You also have to ask yourself - would it be better for me and Twintop to resim our stat weights with VPLC included in the sim? Especially if it affects stat weights? I suppose the answer to that would be determined based upon whether or not you assume a person will have that trinket. If the gear list is leaning towards having a "hardcore raiding audience", then I guess it might be worth it to re-evaluate the stat weights we are using...
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