[Betlis, Switzerland] Vipera aspis? Natrix helvetica? Coronella girondica?

None of the above. It's a Smooth snake - Coronella austriaca - Harmless :)

My kids found this specimen just outside Betlis in Switzerland two weeks ago. The total length must have been a bit over 50cm. It was moving very lazily, but we didn't feel like encouraging it to get its head out in the open, nor waiting for it to do it. This doesn't help with identifying...

Folk in r/askswitzerland are guessing this would be one of: Vipera aspis aspis, Natrix helvetica (more detailed description in French), or Coronella girondica.

Unfortunately I don't have more pictures, nor a reliable measure. I wonder if it is even possible to tell them apart at this point...