VPATH descents are not idle descents on Collins PL 21 avionics. It's following a geometric path defined as by the default descent angle, and what is required to meet any constraints, not whatever path would result in an idle thrust as is usual in commercial airliners. You may need to adjust your expectations. However, it has been acknowledged that the airframe likely needs more drag, but there's no current plan for if/when this'll be implemented.