The existing underlayment is in prettu good shape, except the gaps are excessive and it was installed with (non-ring shank) nails, which have been driven too deep creatign a 'crater' or are working their way out.

I plan to install new underlayment over this but have a couple of (more) questions:

1. What should I use for this undelayment, and what thickness? I'd like to keep it as thin as possible.

2. If I go with the vinyl tile (which I probably will), it obviously will be laid from the centerpoint of the room- how should the underlayment be laid out? I don't imagine 4 intersecting corners of underlayment below 4 intersecting tile corners is the ideal situation. Or is it?

3. Lastly, the dining rom has 1 exterior door. How do I address the new floor height at the door sill. The door obviously need trimming, but I do not know what to do about the threashold.

Thanks in advance