Use a multi-column combobox for selecting account number. Advise not to use spaces nor punctuation/special characters in naming convention.
If ID is designated as primary key, then set combobox properties with:
RowSource: SELECT ID, AccountNum, VendorName FROM Accounts;
BoundColumn: 1
ColumnCount: 3
ColumnWidths: 0";1";1.5
Textboxes can reference combobox columns by index. Index begins with 0. So textbox for vendor name would have expression in ControlSource:
=[comboboxname].Column(2)
If you are not using ID as primary key and saving as foreign key, why have the field at all? If you want to save AccountNum then don't include ID in the RowSource and adjust settings as necessary and AccountNum should be designated as PK.
If you want the same account number to carry forward to next record, use VBA to set combobox DefaultValue property.
Sub comboboxname_AfterUpdate()
Me.comboboxname.DefaultValue = Me.comboboxname
End Sub