VCs are literally in the slide of this post.
The VCs appear to be orthogonal/separate from any of the app data storage/interaction layer, and I see they primarily use the JWT variant of VCs anyway. The data storage/interaction part is listed as DAG-CBOR, so no RDF involved. But aside from all these protocols there's a simple JSON canonicalization RFC here people should use instead of rolling their own, imo: https://www.rfc-editor.org/rfc/rfc8785
Discussion
But they're using the JWT variant if you look at the docs, not JSON-LD/RDF, and the VCs aren't involved with the app datastore, it's just a thing used for signed proofs completely unrelated to Nostr/social network stuff.
You can use VCs as plain JSON, there's no RDF forced on anything: https://www.w3.org/TR/vc-data-model/#json