The cleanest route to a single VCF of all your iPhone contacts is through iCloud on a computer, since the iPhone itself does not offer a simple export all as one file, so the reliable methods:

The iCloud web route, the standard answer: on a computer go to icloud.com, sign in with your Apple ID, open Contacts and with the list showing, click any contact then select all, Ctrl+A on Windows or Cmd+A on Mac, then click the gear or settings icon at the lower left and choose Export vCard. This downloads a single VCF containing all selected contacts, exactly the one file backup you want and it requires Contacts to be syncing to iCloud, which the iPhone does when iCloud Contacts is on in settings.

The Mac route if you have one: on a Mac, the Contacts app holds the same iCloud synced contacts and selecting all then File, Export, Export vCard produces the single combined VCF locally, the Mac native path to the same result.

The direct from iPhone limitation and workarounds: the iPhone has no built in export all contacts as VCF, so the on device routes are either sharing individual contacts as vcf, fine for one but not all or a contacts backup app from the App Store that exports all to a VCF, which works but involves trusting a third party app with your contacts, less clean than the iCloud route. The iCloud web method avoids needing any app.

The verification and the backup habit: after exporting, open the VCF in a text editor or preview and confirm it contains many BEGIN VCARD blocks, one per contact, proving it holds everyone rather than a partial export, then store it somewhere safe as your contacts backup. Since iCloud already backs up contacts in the cloud, this VCF is your independent portable copy, the one that survives even if the account has trouble, worth refreshing periodically as your contacts grow.