I have carpet in the living room, dining room and family room, which abut the kitchen that has vinyl flooring.  These three areas are laid out in an 'L' pattern.  I will put laminate flooring throughout the area.  If I leave the vinyl flooring as is and put the laminate flooring on top of that (making the floor height of the kitchen higher than the other two areas by the thickness of the vinyl), is there a threshold piece that I can use to bridge the difference in height between the kitchen and the other two areas?

I realize that this a bad idea appearance wise, but if all the rooms were tied together with continuous flooring, I would have to establish one base line from which to work.  I plan on using the edge of the step down between the living room and dining room as the baseline, since I prefer a flush nosing at the step down, which requires precise installation of planks at this location.  I suspect from past experience that my walls are not squared to one another, so running continuous pieces that ties all three rooms together will result in a skewed planks against many of the walls of the three areas.   How are such problems resolved?  The only way I can think of reducing this problem is to lay each area separately, so that the problems within each area due to walls being other than 90 degrees at corners are contained within that area, and not compounded by tying other areas to it.  The transition piece would be used to hide the irregularities at area junctions.