Confirmation @ari33bdx
The increase in the number of (inaccessible) messages corresponds to the usual messages of the type « Your package has arrived » when the package is available at the pickup point and « Order delivered » when the buyer has collected their package.
It decreases when you receive a simple message of the type « Question from a buyer » and you have read or replied to it.
(from my side at least)
The pictogram deduction (number) from the messaging works when you use « Smart Messaging » from Clemz.
For those who have a Clemz subscription, open Smart Messaging
Then view the messages by clicking on the icon
A new page opens to the desired message
Once the messages are viewed
Close Smart Messaging
The number has disappeared 
(if you have viewed all messages where there is an ongoing sale)
Another method,
Go to « My Orders »
View messages for ongoing sales on a new page (right-click « open link in new tab »):
Order delivered (the customer has received their package)
The order has not yet taken place (the customer has not yet picked up their package)
Similarly, viewing ongoing orders deducts the pictogram.
There was a change at noon, but I still have « unread » notifications even though everything is OK (and I’ve already checked all conversations for ongoing orders)
Maybe by this evening, Vinted will stop messing with the messaging system 