Do you need underlay to fit a vinyl floor?

no and it would probably be a bad idea as it is likely to move around. Vinyl is cushioned anyway so there would be no point. Underlay is put under carpet to stop the carpet being crushed. Vinyl will restore after being walked on.

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Answer 2:

Underlayment is used on vinyl when installed over a wood substrate. Vinyl will take the shape of the floor below requiring a smooth surface for installation. Without this board, often a floor will telegraph the imperfections below. Additionally the underlayment should have the nail holes and seams between boards patched with a wood filler.