The current logic it is not possible to configure a logical approach to limiting vendor deliveries.
The vendor calendar is a restrictor for the ordering and delivery days. This means if you open a calendar for Wednesday each week D365O believes the vendor is only open on Wednesdays, so you can only send the order on a Wednesday and they can only fulfil it on a Wednesday. In reality this is not a reflection of what is happening, generally you can submit the order on any day, and the delivery day is either when you fit into the delivery schedule for them or what you insist upon due to transport management smoothing. The Wednesday is when you want it, not necessarily the only day the "vendor" can deliver.
Part of the issue is if you have a requirement appear on a Thursday it assumes you cannot order it until the following Wednesday, and if you have a leadtime of 3 days it will deliver the Wednesday after - this is a week after in reality when they order could be delivered. Restricting the order date by the calendar makess little sense, having a shared calendar for order and delivery even less. If you then tick the working days flag it gets worse, the "3" becomes 3 weeks because the vendor is only open one day a week.
This needs to be reassessed to reflect what is happening in the supply chain.