I have a bit of a love/hate relationship with vanity sizing, especially in relation to Levi's Curve ID products. Surely if a pair of jeans states that it's a 30W/34L then the garment should actually measure the same as the label? As a UK 14/16 I am a tad surprised that I can wear anything from a 30W to a 32 without any problem, despite my waist measuring a (couple) of inches more than that. Don't get me started on different measurements for denim washes/country of origin etc, it's crazy. 
My DH's 501s are a 36/32 and that's what they measure.