I've just had a rough experience with VC, and honestly, reading all the negative stories here makes me feel a little better about the loss and the fact that my misery has company.
I sold a pair of boots with detachable feather charms. When I shipped the boots, I took the feathers off and put them in a little plastic bag to include in the parcel. The feathers were delicate and I didn't want them to get damaged in transit, I placed them clearly in the box with the shoes and even had a note explaining why I'd done this. When VC received the parcel, their Quality Control team marked my item as non-compliant and said the feathers were not there. I went back and forth with their CS, only for them to finally admit that they likely threw the feathers in the bin without bothering to look in the box for them! To add salt to the wound, they still haven't sent my now, lower-in-value boots back to me! That's a £115 sale and a pair of ruined pair of boots thanks to Vestiaire Collective's terrible customer service and incompetent quality control team. What's worse is the buyer was really looking forward to them, but so far, VC hasn't updated EITHER of us with what the plan is! Am I getting them back? Will the buyer be offered a lower price? It's like these boots went into a black hole.
I expect you’ve already done all this, and of course you know what you’re doing, but you haven’t already chased this again, from my experience with VC and its quirks, I think I’d give it another a week or two to see if it just gets done (I’ve had resolution emails late with apologies about delays recently, about a couple of delivery issues), and if you haven’t heard any more, I wouldn’t just call, I would email with clear details about the outcome you expect, follow with a phone call, refer them to the email and wait on the line while they read it. It usually helps bump things up, to ensure the problem is being attended to in a (reasonably) timely manner. Emails have to be via the web form in the help pages now, on website or app, so I always screenshot anything I’m sending that way so I can refer back to it — the old email address doesn’t work unless you are replying to one of their emails. I am always really clear, straightforward and determined where necessary, and I find that things get sorted out, usually swiftly, so overall I end up happy, with most of my purchases/sales working out fine anyway, and the small minority with hiccups getting resolved. (If only someone could tell me the secret of getting John Lewis to sort out issues with two Christmas deliveries ... nothing seems to be working there, just endless loops, and they’re normally so reliable!)
If they do get back to you but the resolution isn’t adequate, do the same. You will get a suitable resolution if you are unambiguous with them and correct any errors/gaps/misunderstandings on their part. Hope your issue is sorted out sooner rather than later. Things would undoubtedly be better without waits and prompts but I still find VC a very useful and generally smooth service so I don’t mind being patient with the odd problem. I reckon they haven’t taken on enough new CS people yet and I believe from what I’ve read and what CS has said to me that they are going through another swamped period with increased sales, and, although things are generally getting better, things can still be slow at times, and they haven’t quite optimised the links between departments to expedite and seller/buyer problem resolution yet.
It’s such a shame about the missing feather charms. As regards the packing, I’d say the same as Gabs. I once had a bought item sent with a part missing. They agreed they’d lost it and offered suitable alternative resolutions, so I would assume they will get around to this with you/your buyer. I think in what I imagine is the giant warehouse there is rarely or never any chance something missing will turn up. So I do more or less what was suggested above. I try to imagine in advance all the possible ways the unwrapping could go wrong in the warehouse and then I completely foolproof my sold item packages, putting unmissable notes in and literally tying or pinning things together, in bags if necessary to avoid damaging the items. It sounds like you did most of this anyway but I take it to obsessive levels! I sometimes also take quick phone pictures of what I’m packing and how, just in case of question.