You technically don't need the PR Portal, I did the same trip, same airlines a couple of months ago but to EIS instead of VIJ.

Delta never asked for evidence of the portal. When you get to SJU, have your connecting flight information available at baggage claim. Just show it to the National Guard they will direct up to the Cape Air counter to recheck your bags. We had no luck even as a Million Mile and Diamond Delta member checking bags thru to Cape Air. No big deal to recheck them but I had planned a long layover.