Repealing vacancy deregulation is crucial to saving the existing stock of New York City's affordable housing. That's because roughly 95% of all apartments removed from rent stabilization are de-regulated when they are VACANT. That means that the last tenants' incomes are irrelevant to deregulation. (Only about 5% of apartments are taken out of rent regulation because of the owner's annual income of $200,000 or more for two consecutive years and a monthly rent of $2500 or more. In fact, the median household income for rent stabilized tenants is $39,000/year.)
For you legal buffs out there, many of the rent laws governing New York City have been retroactive: the 1947 rent control law put buildings with 6 or more units built before that date into rent control; the Rent Stabilization Law of 1969 stabilized apartments in such buildings built before the law's date; the Emergency Tenant Protection Act of 1974 did the same for buildings built before 1974.