I don't have the PMDG installed but let me answer some of your general questions.
1. When you receive instructions to turn(Radar vector) you don't put it into the FMS. You select HDG on your Autopilot control panel. If you're receiving instructions from ATC, the FMC does not matter. When ATC tells you to go direct to a fix or intercept your course, then you're back on the FMC. You can only do one at a time.
2. Your rate of descent is not controlled by the FMC. Often the FMC will display a TOD point based on a descent angle of 1-6 degrees, normally set for 3 degrees. The distance that this point is displayed depends on cruise altitude or more accurately the difference between cruise alt. and airport elevation. You can start you descent whenever you want, it's just a guide. IFR, ATC will tell you when to descend. The descent rate is set by you via the AP control panel.
3. The FMC's job is to get you to the vicinity of the airport, not to land. Once you get within approximately 20-30 miles of the airport, you should be on a heading so that you can intercept the ILS LOC. There are some exceptions but I don't want to confuse you. The altitudes in the FMC or just a guide, you control your altitude via the AP control panel.
Many people give the FMC more credit than it deserves. On a real flight in a busy airport like New York, you may fly for 15 or 20 miles before you even use the FMC. At the destination, you go to a heading 20-30 miles before the airport. Of course there are exceptions but more times than not, that's what happens.
🙂