The user would first be presented with a continuous form displaying a list of vendors
I'd be more inclined to use a combobox or listbox
User is presented with a continuous form displaying all of the products with these columns: ProductName, VenProdDateBegin. If the selected vendor already provides some products, the VenProdDateBegin (from tbVenProdLink) will be entered into the VenProdDateBegin field for that product (so the user can see what the selected vendor is already providing)
I'd be inclined to use a listbox with 2 columns. the second column showing that its already provided.
For products which the user wants to assign to the selected vendor, the user will enter today’s date into the VenProdDateBegin field.
I'd use a multiselect listbox. After making selections then click an Add button to add a date to the selected products with a null begin date( the null begin date criteria is in case they select a product already being provided). Then requery the combos/lists or close the form
If this helped, please click the star * at the bottom left and add to my reputation- Thanks