RE: Buyer group defaulting on purchase order
You don't need to specify a buyer group on the vendor group/vendor master.
Many companies prefer to do that because typically there are specialist buyers for specialist items from specialist vendors i.e. if I am buying an Engineering item then I will likely be part of the engineering buyer group not the travel buyer group.
However if you have a buyer group by department and the engineer bus everything for the engineering department then your buyer group cannot defult by vendor or item.
Approved vendors by item restricts the choice vendor.
Buyer group on the other hand does not restrict - its more to aid filter sort and report.
Consider the implications of changing the default ad hoc by order - it depends on your reporting/filtering /planning/forecasting etc, whether that is feasible - does it even matter whether you have a Buyer group on the order? Who else might use it?
For example see these posts for useful tips timsaxblog.wordpress.com/.../use-the-buyer-group-on-production-items-in-dynamics-ax
organicax.com/.../buyer-group.
Workaround suggestions:
Make it a mandatory field but don't default a value - if you wish then also customisse to look up the buyer group from the user id
Don't default from the vendor master instead use the copy order feature where the purchase pool/or the buyer group is already manually defined in effectively a template order.