Newtonian energy diagram for flat matter + vacuum model. Since there is a "gap" between the peak of the gravitational (dashed) curve, and the zero "total energy" line, the model coasts over the top and onto the "downhill" accelerating stage. Notice the time of coasting (top of hill when velocity is constant) occurs before the time of density equality (where green and blue lines cross). This is because for a given density, the vacuum has a slope that is twice that of matter.
Figure: Whittle's (upcoming) undergraduate textbook.