How much farmland would we need to produce enough food to feed 7 billion people?

Were now beginning to utilize vertical farming techniques. It's not 

perfected yet but given enough energy to run pumps and provide 

light we may be able to feed thousands of people off of one acre. 

I have heard that a rule of thumb was one acre can feed one person indefinitely but there are many variables. Groves of citrus will take more land and so will livestock. I have also seen articles about hydroponic set-ups where a single acre could feed 60 or more people. I guess it will depend on how much energy and money we are willing to devote to food production. 

After doing a bit of math and if were talking fiction here where farming is being carefully managed for maximum production and not for profit, Kansas, Nebraska and Idaho together could probably feed the world. If the world were vegan (no livestock).