laying ceramic tiles over floor boards with existing vinyl tiles

Hi,

We're in the middle of a laundry renovation and I have a few questions:

1. The laundry is a suspended concrete slab and we are keeping the original dimensions. We are having tiling installed over the slab. What sort of waterproofing is required on the slab and the walls behind the tub?

2. We have cut a new door way through to the kitchen. the flooring changes from floorboards to slab under this new door with a small change of height (5mm). What is the recommended height of the step up into the laundry to help stop the water from the laundry running into the kitchen

3. We would like to install the same tiles in the laundry and kitchen. The kitchen currently has lino installed over masonite sheeting and very old vinyl tiles stuck directly to the floorboards under that. The lino and masonite will come up very easily, but the old tiles are stuck hard and and are very brittle. Is it ok to lay ceramic floor underlay over the old vinyl tiles, ensuring that the underlay is nailed properly to the floorboards? I'd love not to have to scrap those old tiles off piece by piece! Saying that though I still want a good finish.

Thanks any help appreciated.

Steve.