Hi! I think others have given you feedback about the benefits of buying it from Birks (no interest on credit card, refund). Those are the obvious benefits.
For me though, I prefer to go directly to a boutique, especially for long term items such as jewelry that will require aftercare, even a simple cleaning. The main reason is that they have my purchase in their system. VCA seems really picky about repairs and maintenance, and I have read in some cases in the US, if you did not purchase your piece at a VCA boutique (e.g. they cannot find you in the system), you need to bring your COA to show proof. That would just annoy me if my "authenticity" to repair or clean a piece depended on my little COA. I would prefer they look my purchase up, and yup... they see it's my piece.. authentic, and they service it.
As for a refund, I know VCA says no refund, credit only. From my own experience, that's not a hard rule, and I would think if you are undecided about two pieces, they will have some leeway, knowing you will purchase one.
Birks carries VCA today, but they may not always. This is already true of Neiman Marcus... there are some NMs in the US where the VCA boutique closed down. Related, Chanel also shut down a lot of leased boutiques at Saks. This happened a few years ago. And with jewelry, it will always be something especially as we age and wear our pieces... My fingers got fatter, resize please. My bracelet is all dirty, clean please. My earrings are too tight, loosen please.
SAs come and go, leased spaces come and go, but the VCA maison remains.
Just my two cents for another point of view.
Good luck to you!