Removing tarry adhesive from concrete floor


Hi,
I am refurbishing a small under-stair toilet. I have removed the vinyl tiles, which were cracked and broken, and underneath is a very thin layer of tar or bitumen which looks like it was use to adhere the tiles. When I say thin, at its thickest it's no more than 1mm (if that) and mostly it is barely more than colouring the concrete. The floor is overall fairly flat and even.
I would like to lay sheet vinyl flooring. How should I prepare the floor?
I have tried scraping off the bitumen with a hot-air gun and scraper but it either has no effect (not thick enough) or it just pushes the tar around.
Can I lay the vinyl straight on to the bitumen or must I remove the latter? Could I paint the bitumen/concrete with PVA to seal it? If I put down a thin layer of self-levelling screed will this react with the bitumen?
I have tried Googling but unfortunately I get every answer from 'do nothing' to 'remove several mm of concrete with a special machine'!
TIA, Pete
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You can lay the vinyl onto the tar but I would be tempted to use a sheet of paper between the two.

PVA will do nothing to the bitumen.

I doubt they will react. I also doubt the screed would stick properly.
I wonder if this really bitumen or just glue from the tiles?
Adam
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