It will be the 16th, it will be around the time you bought the item.
In vinteds T&Cs it says it could be up to 7 days after this date though.
It will change to say on the transaction screen that they are waiting confirmation from the couriers that it has been sent and if not they will refund you (or words to that effect)
Don't cancel yourself as you will get negative feedback, let vinted cancel it and the seller will get negative feedback for not sending.
Also don't change the vinted negative to your own as that then allows the seller to leave negative for you.
I had one yesterday which I was convinced was not going to be sent and the seller sent it just before I think it would have been cancelled.
The seller will be getting reminders to send.
Are they active on the site as there are quite a few accounts where the seller has not been seen for months or even years, have they recent feedback.
Have you checked the tracking if they have downloaded the label as I have had a few items that I have bought recently where the tracking has not updated immediately - royal mail and evri.