If I was looking at a house to purchase, I would consider sheet vinyl to be the lowest quality of flooring possible. It's a low investment and if you are replacing old sheet vinyl, you aren't going to "lose" money. But it's not going to help resale value. More on reddit.com

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In all honesty it looks to me like it was split by someone awhile ago to repair it. And if it wasn't you I'd say the previous people had someone repair it and it's failed over time.

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Floorboards are not great as often they either have small cracks or are slightly uneven (or both). The simple fix is to buy some really thin plywood (~3mm is fine) then screw and glue it into the floorboards. Use short screws like 20mm and you won't go through the bottom of the floorboards (which are probably at least 18mm) so will have 0 chance of hitting any pipes etc. If the floorboards are very old and uneven you would have to make the ply a bit thicker but the same principle applies, basically the ply thickness needs to be at least the same as the most uneven "jump" between 2 adjacent floorboards, it's very rare that 5mm wouldn't be big enough. If they are T&G though, they should be pretty even so ~3mm should be fine Really if you are doing this you want to go under the toilet and sink with the ply if you possibly can, it should be so thin that you can just unscrew the toilet from the floor/wall and slide it straight under and re-tighten without having to touch any of the pipes (same for the sink). In this case you may as well put the vinyl under as well - it will look better, stop any water from getting to the wood and make cleaning easier. More on reddit.com