When you want to add Contacts then map the VIDS to the addContacts and the value for VIDS needs to be like this:

{"vids": [1,51]}

Here is another way of showing what it needs to look like if we are using a SQL query as a source for some sample data columns:

select '1' as ListID, '{"vids": [101]}' as VIDS, '8354852' as portalID