Do not confuse a vent with a drain...if it only serves as a drain for the vanity, it could probably be reduced to 2". But, it probably also has a connection to the toilet and tub, so, no, it must remain that size. And, if it is actually a vent for things below, it wouldn't pass code today, as you're using it as a drain and wet venting between floors is no longer allowed and needs to be at least that size to work well. Last kicker is that even in your own home in MA, it's illegal to do any plumbing...a pro would know what is required and how to do it properly. Many states allow a homeowner to do plumbing in their own home, but they all require a permit and an inspection. Failure to do so could get expensive and potentially dangerous.