MarsSeed [1] filed a deletion request for vala-language-server-alpha [2]: Discontinued since 5 years ago.
Use the much newer vala-language-server{,-git}, available on AUR. [a]
[a]: https://aur.archlinux.org/packages?K=vala-language-server
[1] https://aur.archlinux.org/account/MarsSeed/
[2] https://aur.archlinux.org/pkgbase/vala-language-server-alpha/
