I have experienced same problem with my Andersen 200 narrowline double hung windows circa 1981. The bottom sash is vinyl clad with wood interior with the ends exposed wood. The wood is almost completely gone from rot. I only have two windows with this problem. They are on the south side of my house and are the most exposed to the weather. The rest of the windows in my house are under a significant overhang and protected from the weather so no problem with those yet.
On one of the windows I removed the interior trim of the window covering the bottom sash this exposed the front side of vinyl clad sill. I drilled 8 holes using a 1/4 inch bit in the front of the sill. I sprayed window and door foam filler in all the holes. It filled the vinyl sill pretty effectively. Quite a bit flowed out the exterior corners so if you do this either cover the corners or have something catch the foam as it spills out. After the foam dried, I cutoff the exposed foam that expanded out the holes. Then I put the interior trim back on covering the holes. This seems to have worked.
I’ve read on other forums that foam might prevent proper drainage of the window This is weird how Andersen might have designed a window that requires drainage around a piece of wood. I’m not sure if the foam will cause a drainage issue or not. I’m waiting for some heavy rain storms to try and inspect before I do my other window. Has anyone else tried this solution and have had problems with drainage or any other thoughts?