How to sync a local PC vault to my phone via WLAN?

That's quite a challenge, to get some files automatically copied from your PC to your phone. For this tedious task all those cloud providers popped up in the last 10 years. I suppose, the sync provider that comes nearest to what you want is something like Owncloud: https://owncloud.com/ or Nextcloud: https://nextcloud.com/
It's total overkill for just sync a 1Password vault, but as far as I see it, no simple "just sync this directory" solution exist. As far as I see, even the once common sync tools for mobile devices vanished, because these days you usually sync via public cloud providers such as Onedrive or Google drive.

You might rethink your position towards 3rd party clouds. They are not insecure, otherwise you would read about it in the media - it would be a huge scandal nobody could miss. If it comes to 1Password, read the security design documentation they made and see how they made it nobody except you, not even they themselves, are able to decrypt cloud-synced vaults.

Even more, the more you custom build some own sync, the more insecure the whole thing gets, because you as a single person, probably a nonprofessional if it comes to IT security, cannot build something as secure as a whole team of professional software developers. The risk of you accidentally building some security leak by configuring something wrong is quite high. The risk of the same security leak by using some standard sync is rather low.