Why are believers to work in the Lord's vineyard during this life?
Believers work in the Lord's vineyard during this life because they are to be expecting rewards someday. That reward is at the bema seat, where believers come before Him for rewards, after the first resurrection, when all believers of all ages rule and reign with Him for a thousand years.
Les explains that one future event is the marriage of the Lamb, and believers will come before the bema seat for rewards. That is after all why we are to work in the Lord's vineyard in this life here: we are to be expecting rewards someday. He then points to verse 6, where after this first resurrection all believers of all the ages are going to rule and reign with Him for a thousand years. Paul deals with resurrection in plain simple language in 1 Corinthians 15, using common ordinary everyday events to describe it. There Paul writes that Christ is risen from the dead and become the first fruits—plural—of them that slept, or of those who have died.