Im installing Dura Plank made by Congoleum over 900 sq ft of wood subloor approx 3/4 inch .The old flooring is a combo of carpet and sheet vinyl. The vinyl is on paticle board 7/16 inch thick.. What is the proper method of matching the two different heighths.How should I Level the floor after I match the heighths

You should tear out the carpet, not try to lay flooring overtop of it.

Oh yes definatley the carpet is coming out. But That will bring me down to the sub floor, and the vinyl , which iwould like to leave down , is 7/16 of an inch higher than the subfloor it sits on.Can I nail down 7/16 osb on the 3/4 inch sub floor,so it matches the vinyl height

I've been through this process many times, none were any fun. Looks great when you are finished but it is miserable work!

When I pulled the carpet off the bedroom floor, the remaining particle board was so pitted and rotted that I just went back to Home Depot and picked up new wood panels.

Ripped out all of the sub-floor and replaced it with the new wood.

Also, they make floor leveling compound that allow you to pour the compound onto the floor and let it seek its own level.

Always, level the seems, then make sure the floor is level. Do this with a level. As you slide the level over the new sub-floor, look for any open spots under the level. If you see them, use the frame of the level to smooth out the leveling compound. Also, make sure you level is level by paying attention to the bubbles.

It is so very critical that you get the sub-floor a smooth and level as possible.