How do I prevent damage coming through from below?

Our kitchen was extended before we purchased the house. The extension was then Marley tiled to match existing. By 2001 the Marley tiles in the extension were "bubbling" so we removed them and re-screeded the area. We then put a good quality vinolay down. In 2003 we noticed a small, dark spot which we tried to remove, but this has now spread right across the line of the extension, ruining the vynolay. We assume that we need some sort of sealant before laying new flooring. Can you help please? (A carpet fitter we spoke to said he thought the Marley tiles had been laid with bitumen and it would keep coming through - is this right?)
Thank you

John Hobart
August 2008