A VAS Provider is a company Apple has approved to issue NFC-enabled wallet passes, and to bring merchants through the entitlement process that makes those passes readable at a terminal.
Apple VAS is enabled by an entitlement Apple applies to an Apple Developer Account. Without it, a pass can still be issued, added to Apple Wallet, updated and scanned as a QR code or barcode; what it cannot do is carry an NFC payload. The entitlement is what turns a working pass into a tappable one, and Apple grants it once it has reviewed the use case: what the program is, where the taps happen, and what the pass identifies.
That is the work a provider absorbs. Stell is a VAS Provider: we prepare the entitlement request alongside the merchant, describe the program in the terms Apple assesses, and stay in the exchange until the entitlement lands on the account. Because Stell registers the pass type identifier under the merchant's own Apple Developer Account by default, the entitlement lands there too, and it stays with the merchant the way the identifier does.
The Android counterpart works differently. Google Smart Tap is enabled through the issuer account rather than through a per-merchant entitlement, so the two platforms are approved on separate tracks even though the tap at the till feels identical.
Use cases differ, and so do the questions Apple asks about them. Tell us what yours is and we will tell you what the entitlement request looks like for it.