One of a vendor API documentation mentions that their API calls require to use HTTP Basic Authentication scheme, i.e. user:password Base64 encoded but, their token API (Login equivalent) documentation mentions that "..this service implements OAuth 2.0 - Resource Owner Password & Credential Grant"
Isn't HTTP Basic Authentication different from OAuth ?
Yes, HTTP Basic Authentication different from OAuth 2.0. However, the Resource Owner Password Credentials Grant utilizes Basic Authentication Scheme within the Authorization Request for the Client's credentials as described with section 4.3.1. Authorization Request and Response
The Resource Owner Password Credentials Grant is typically used to convert legacy systems to OAuth 2.0 and no more secure than Basic Authentication Scheme.
Resource Owner Password Credentials Grant is intended to be used when no other Grant Types are available and ONLY when there is a high degree of trust between the Resource Owner and the OAuth Client .