OK I do no get why you need to level this. To be clear if you need to the tub doesn't care. Clean/prime area and pour leveler. No issues there.
However there isn't that much of a slant and I would only use leveler if there were holes/pits. Also the slant is going in a direction that is perfect. Your baseboards are the only way to really tell of a slant this small and since they are on perpendicular walls they should be relatively level.
When installing vinyl planks you don't need level though, you need flat. That bathroom looks flat. I see no reason at all to do this. (I have installed vinyl planks on basement bathrooms 20+ times - only one used leveler and only because bathroom was huge - 10x22).