Airbnb is growing, but I don't fully trust either them or VRBO.
I am fortunate in that I have a personal longterm relationship with the villa owner from whom I rent directly.
Airbnb and VRBO are too anonymous for me, with potential risks. Of course Airbnb (and VRBO) have ways to mitigate those risks - at a cost (hello agency fee, LOL). If I did not have the relationship that I have, then I would still be doing agency rentals.
As to Agency rentals, I would only ever rent through an agency which has a physical presence on St Barth. At least one such agency has apparently released their entire inventory to just about anyone who wants to be an online agency. I only want to do business with the on-island agency which has a direct relationship with the villa owner.
So, what does an agency bring to the table other than a service charge?
The most important thing is that you know who you are dealing with, you have a relationship with a reputable vendor, and you're paying a known entity.
The major agencies also offer concierge services. They can help with anything from booking your air travel (the best ones have an air desk), to a rental car, to restaurant reservations, to a boat rental, to arranging a babysitter, to arranging a picnic - lots of stuff, and much more than I have mentioned.
Agencies are also your go-to person for a villa issue, anything from something small such as "this villa doesn't have a corkscrew" (been there) to something huge such as "the septic is backing up out of the shower drain" (been there too). The corkscrew was a 60 minute fix. The shower drain was a move to a new villa, which is unlikely to happen with Airbnb. My point here is that a good villa manager can fix a small issue, but a major issue may require villa inventory which you won't get through Airbnb.
Another thought about Airbnb is the level of information and support which a renter may receive. For you, a many-year visitor, that's not a problem. For first-timers, I hope that they read SBHOnline religiously before they travel.