Call a local home improvement contractor who specializes in roofing and siding. We can’t see how your vinyl window was installed from the pictures you provided. As others said lack of aluminum drip cap might be a cause. Also there is no vinyl J channel around the window frame in the pics. Does the window have an integrated J channel built into the frame? If it’s damaged at the head of the window, This could also be the cause. Vinyl siding is incredibly easy to work with. It Doesn’t look like you need to replace any siding at all. You need theinstallation details inspected for defects or missing components. It shouldn’t be very expensive to remedy the issue.

You are correct. That’s not broken siding it’s actually what is called a mull strip cap. When manufacturers join two or more windows they install a mull strip cap over them where they are joined. Basically it covers the hollow vertical vinyl piece between the two windows.
Lack of alum. head flashing seems to be the most likely cause.
Be very glad it’s in a vinyl siding area if it was stucco or Stone it would be quite a bit harder to repair.
Did you buy your home new? If so call your builder ASAP. New construction latent defect laws vary by state but 7-12 years are common in my area.

Please read my first post.
Also It’s hard to tell from the pic but it looks like the siding is caulked directly into the integrated J channel at the head of the window. The fix may be as simple as recaulking with a quality silicone.

If it never leaked for 14 years and this is completely new, most likely it’s failed caulk/silicone or a deterioration of the vinyl windows integrated J channel. I’ve seen vinyl “welds” fail where the various pieces are bonded to each other. UV damage sometime is the cause.
Also keep in mind that weather has become much more extreme than it used to be. Most areas get 100 year storms every year or every few years now. Unless your house is built like a boat water will find a way in during heavy rain and very high winds.
OP please Keep us posted.
I’m curious to see what you find.