You can get lucky, personally I would recommend to pay with a credit card and as soon as you get the item have checked by a reputable jewellery store as especially with small stones they do not check for flaws or clarity and their verification seems to be hit and miss. I understand that getting an independent 3rd party verification is an added expense, but less expensive than ending up with a fake. In case you pay with a credit card, it will take a while to get your money back, but if you can show them that what you got was not what you bought. the 72 hours Vestiaire gives you doesn't apply to your credit card company doing a charge back.
I don't want to talk Vestiaire down, I bought some really amazing items through them, but their verification is hit and miss, I had to return items (they didn't want to take them back as it went through their verification, I have a rather low opinion of their "in house experts" and trust an independent 3rd party authentication more) on the other hand, they flagged items I bought as having a "small problem" when it was clear the item was new and the crease was from the item being folded. I am not sure if they roll the dice or it depends on their mood.
What shocked me the most was that when I forced a return through my CC company as a 3rd party authentication said the item is definitely not authentic and gave all the reasons (including that such a serial number doesn't fit the item, stitching, etc.) I saw it listed again with the same pictures but as shipped directly from Vestiaire with verification and speed shipping. So I won't buy anything that they list, the risk seems to be a bit high.
I think you can buy great items, as long as you are careful, use a credit card, and if you have the slightest doubt have it authenticated again by a reputable independent authentication service.