Nextcloud is an open-source, self-hosted file share and communication platform. It's a suite of client-server software for creating and using file hosting services. Nextcloud provides functionality similar to Dropbox, Office 365 or Google Drive when used with integrated office suites Collabora Online or OnlyOffice.
Coming later
In my experience, most Android "Contacts"-apps will not be using vCard3, but rather vCard2.
So if you export your contacts as a .vcf-file you need to convert them to a vCard2 to be able to use them.
For that I found jowave/vcard2to3