The Health tab on your weatherlink.com station account should show whatever health information is available for your station.
From memory, the Vue/VP2 transmitters don't report a battery voltage but only a good/bad flag - obviously better than nothing. So this will just give you confirmation of why the email from Davis was triggered.
I'm afraid that if the battery life seems to be getting progressively shorter, the problem is likely to be the supercap and not the solar panel. For most users, the simplest option is then a new Vue ISS unit once the battery life becomes too short to tolerate. Yes, you can buy a new wiring harness but there are complications to that (eg has your Vue got the solar panel hardwired or not and there's not a lot of cost-saving vs a complete new ISS).