Residential vacant land - what kind of people (demographic group) usually buy this?

Residential Land buyers can be anyone.  From the inidividual homeowner want to buy a site to build on or the contractor that wants to buy up some land and rezone it for multiple dwellings.  It depends on the area and the market conditions.

Land and lot buyers in our area are a wide range of buyers, statistically speaking.  They are anywhere from 20 to 80 years of age.  They are blue collar and white collar workers. Some are lawyers, some are farmers and some are just out of school.  I find most of the buyers for this type of transaction are seeking to buy land and build on later in time. Anywhere from one year to fifteen years is a typical timeline. Some are seeking long term investment with an ROI years down the road as well. Some are seeking a safe investment that will not disappear or evaporate with todays economy.  Unlike stocks, bonds etc... real estate will always be there.  Sure the value will fluctuate as with any investment, but it will never be zero, like some investments in the recent past have achieved.
Owning land is an investment a diverse group of people make.  If you are planning on selling then I would strongly suggest a real estate agent to promote your property and have it shown to the world on the internet and MLS system.  If you are a builder then, well, in one word, patience.