When to Unicom or CTAF when ATC "Center" is active

I am new to VATSIM, have used PilotEdge in the past; If there is an active ATC “Center” and I want to land at a “Non-Towered” airport with a CTAF; do I need to contact “Center” to get clearance? In the real world or in Pilotedge you use CTAF to communicate intentions, initiating call with Airport “Traffic”. What is the procedure on VATSIM. Unclear in Code of Conduct.

If the airport is uncontrolled (as in, there are no ATC positions at that airport) you use CTAF for all relevant communications. Getting your IFR clearance at these fields would normally be done by calling up the responsible center controller, they will give you your clearance and likely tell you to call when for release at the holding point. When inbound under IFR, you will also get something like “change to advisory” at some point and then switch to CTAF. All in all pretty similar to real life (and the same goes for phraseology).
If there is no controller online, just proceed as if you had gotten the appropriate clearances and when no longer/not yet in the vicinity of the aerodrome, switch to VATSIM’s global “Unicom”, 122.8 (please also note that the use of real world CTAF frequencies is currently a trial limited to only a few places - if you are flying outside of these places, the global 122.8 “Unicom” is used for all communications when there’s no ATC, even the ones that would take place on a CTAF or similar IRL).
If you are unsure whether a controller may be responsible for the area you’re flying in or just in general unsure if you need to talk to them, check a map tool such as VATSIM Radar, and if still unsure after that, just call up the controller you think might be responsible for your flight and ask them.

Thank you for the response. to be clear if the airport uncontrolled, Non-Towered Airspace, then the appropriate CTAF should be used for communication for that space as in the real world. If the airport/airspace is a “Controlled” then the appropriate online ATC position should be used for VFR flight correct?

Yup, that’s basically correct.
And if it’s a towered airport that has nobody online right now (also nobody providing topdown coverage), you would also use the CTAF if it’s a part time tower and the tower frequency if it’s a 24/7 tower (and if there are multiple tower frequencies, take the one that’s listed first on the sectionals).

So that I understand, say for example that Albq CTR is online, If I am taking off from a towered airport such as KCHD, Chandler Airport, my home airport, then I should contact Albq CTR for Taxing and Departure approval as if I was talking with KCHD Ground and Tower, is that correct? Same would be for arriving at a Towered Airport such as KPRC, Prescott airport.

Correct.