The kingdom of God simply is the agenda or the purpose of God. And since the beginning of creation, He created man for His pleasure and to serve in His kingdom by working for Him. The evil one sought to destroy this kingdom of God by leading man to rebel against God to create his own kingdom and force man to work for him.
However, Jesus has paid the price and created a reconciliation avenue for man to be restored in working for God. Since the end of the age is yet to come, you can still see the effect of the devil’s kingdom created in the form of a world system to keep man away from working for God and His kingdom. This world system is what is referred to as the marketplace whereas the kingdom of God is referred to as the vineyard in the parable.
Serving your purpose in life as a believer is when whatever you do brings glory to God (the vineyard owner) and pushes the agenda of the kingdom of God. When you are not working in the kingdom of God, you are in the marketplace no matter your career, position, status or prominence in society. If God has few or no records of your contribution to His kingdom, then you can be referred to as idle (serving no purpose), just as the landowner referred to the people in the market-place. It is only those who responded to the invitation and worked in the vineyard that had rewards from the Landowner.
You can be a teacher, a doctor, a fashion designer, a caterer, a student or a driver and work just for the salary you earn and how you can be comfortable in life and as such do just anything to achieve your interest and that ends it. You can also earn your salary and eternal reward in addition by doing the same work in any of the fields mentioned above if you work with the Kingdom of God in mind.