This was answered earlier today in the comments that are now missing:
SET @summary = 'CCC.dbo.spVendor_Active_Update: Vendor.IsActive (old): [' + Convert(varchar(10), ISNULL(@oldActive, 0)) + ']; (new): [' + Convert(varchar(10), ISNULL(@Active, 0)) + '];';
Adding ISNULL was necessary to convert null bit values to varchar.