Hi Carsten,
A previous group implemented the time-dependent business partner settings and with good reasons. We did thought about removing the flag for time dependency (just to test MDG-S in a copied system) but unable to do so with the volume of customer records created. It looks as if configuring vendor as a custom MDG could be diificult and not the best decision. But what sort of issue have you encountered? Can you provide more details?
Best rgds,
Rom