Where would you use V/S instead of Lvl chg?
Just remember in Lvl Chg, your tail plane is only controlling the speed. Nothing else is being controlled, other than the thrust levers being closed. (in a descent!)
In v/s the autopilot is controlling your rate of descent,
and a speed which may or may not require auto thrust to activate, and the thrust levers to move.
If you want to use v/s but want to be sure the thrust levers remain closed, then select your desired v/s, but wind the speed bug back to 20 or 30 kts below your indicated speed. (Or even back to your next target speed!)
Excess speed is no longer controlled, because you are commanding a rate of descent! (This is very important!!!!) However you can be sure the thrust levers will remain closed. You can use speed brake if the speed starts to run away, but obviously that will have no effect on your rate of descent!
I found this particularly useful on Airbus aircraft with non moving TLs, but it works just as well in the many Boeing’s I’ve flown.
Oh, and never forget. You’re still learning so never beat yourself up if it doesn’t work out. That just makes it ten times worse! Just smile, and take it on the chin. Soon enough it will just become second nature and all these calculations and ring drawing will be superfluous!