Subfloor Water Damage
As you noted, it's possible the water came from the front porch, ran under the door sill, across the rim joist, and seeped across the subfloor, possibly between the layers, before showing up in the basement. Because any and all of these may have been wet long enough for the subfloor to rot, they should all be checked. 

I'd start by probing all of the areas around the rot with an ice pick to find the extent of the soft, decayed area. If you can unscrew the sill or lift part of the vinyl planking you can come from the top. You can also probe from the basement up, starting in the obvious rotted area. Since you don't own the home yet, I doubt they'll let you drill exploratory holes or do other destructive tests, but some careful poking may tell you more about how much repair is needed. After repairing the wood, consider how toy would keep water from getting to this area again... pouring a new porch, adding an awning, grading the front yard, etc. Consider those costs when deciding whether you want to ask for concessions or accept the risk there may be more that you haven't seen yet.