When ATC requests you to descend, you cannot wait until you reach your Top of Descend. This can only be done, when you hear the phrase “when ready, descend to FL…”
I just finished I flight on vatsim. The controller asked me to descend, but I was still a few miles away from my Top of Descend. I did what @JasonoNovi described. Additionally you will see the diamond, which indicates that you are below your vertical path. When you decrease your rate of descend, you will eventually pick up your descend path and VPATH will take over from VS