Lola,
You are asking a lot for a place like this. It is hard to provide that much information in a few sentences.
I have done snack and soda vending, bulk vending, amusements, pay telephones and did it for 9 years became one of the top five vending companies in my state and did it all within 4 years of start-up.
However, I still did not take home a pay check. Had way too much debt to fund the growth.
To narrow it down to one type of vending is hard. They all made money.
Here are a few tips:
Pick a vending meathod in which you can compete. You need to be better than the next guy. Or offer something that no one else is offering.
Only buy machines that you can pay cash for: NEVER, NEVER, NEVER finance vending equipment. If will suck all the profit out of it and the only person that will make any money is the finance company.
Buy good quality equipment. Metal is stronger than plastic no matter how you look at it.
Stick close to home: with the price of fuel you don't want to drive long distance to fill/ collect your machines.
I hope this helps. thanks