I'm currently remodeling our entire kitchen and would like to replace our vinyl floor with hardwood (mirage). The kitchen subfloor is 1/2" plywood with 5/8 particleboard on top. We have solid 3/4" 2 1/4" oak in the rest of the house (installed perpendiculiar to joists) and would like to use the same. The problem is it seems that our builder used 1/2" plywood on 2X10 joists 16"oc on the entire house including the hardwood sections. All todays prefinished flooring co. seem to recommend minimum 5/8" ply. The old hardwood floors don't squeak and seem to be solid (installed over 20 yrs ago and have been refinished once). My questions are:

1- Should i remove the vinyl and particleboard and lay prefinished flooring in the same direction over the same 1/2" plywood to make a smooth transition between the two rooms? What problems could we have considering the rest of the house has been done that way for so long?

2- Should we remove everything down to joists and install 5/8 ply and go by the book? How do we handle the small 1/8" height difference?

Thank you for any advice or recommendations.

Your situation presents an interesting dilema. IMO, since you already have 3/4" oak floors which, according to you, are performing well, I would go ahead and remove the particle board and install the new floors over the 1/2" ply. The reasons you can do this are:
1) You will be installing the floors at a right angle to the joists.
2) The joists are 16" OC
3) You are using 3/4" solid T&G flooring.
In this case, after laying down your 15 lb. asphalt felt paper, chalk out lines to mark the joist locations. You will want to try to blind nail through the plywood and into the joists. This doesn't matter much with 3/4" plywood subfloors but in your case, it will make a difference.

I second that. I have installed oodles of jobs just like yours. It is plenty solid. I don't like installing anything but strip over 1/2 in, though.