calling a web service with API keys

If you were using "Call Web Service" smart service - it implies that you were consuming a SOAP based Web Service - and were authenticating using username/password as you mentioned.
If the new vendor has a "API Key" to authenticate, it means the authentication mechanism is different. Can you let us know if it is SOAP or REST based web-service that you are going to consume?
As far as I know, in RESTful webservices, the API is generally passed as a query parameter or as a header. Most of the webservices use a query parameter for the API key.
So, it depends on - whether your webservice is SOAP or REST based, and whether the API key needs to be passed in the Query parameters or in the Headers. You will need to ask the vendor who is providing the web services for more details.

If it is a REST based webservice, you can consider using the "HTTP Query" smart service or the a!httpQuery() function on SAIL forms.

If you were using "Call Web Service" smart service - it implies that you were consuming a SOAP based Web Service - and were authenticating using username/password as you mentioned.
If the new vendor has a "API Key" to authenticate, it means the authentication mechanism is different. Can you let us know if it is SOAP or REST based web-service that you are going to consume?
As far as I know, in RESTful webservices, the API is generally passed as a query parameter or as a header. Most of the webservices use a query parameter for the API key.
So, it depends on - whether your webservice is SOAP or REST based, and whether the API key needs to be passed in the Query parameters or in the Headers. You will need to ask the vendor who is providing the web services for more details.

If it is a REST based webservice, you can consider using the "HTTP Query" smart service or the a!httpQuery() function on SAIL forms.