You can invite guest users to the directory, to a group, or to an application. After you invite a user through any of these methods, the invited user's account is added to Azure Active Directory (Azure AD), with a user type of Guest. The guest user must then redeem their invitation to access resources. An invitation of a user does not expire.
The invitation will include a link to create a Microsoft account. The user can then authenticate using their Microsoft account. In this question, the vendors already have Microsoft accounts so they can authenticate using them.
In this solution, we are creating guest account invitations by using the New-AzureADMSInvitation cmdlet and specifying the -InvitedUserEmailAddress parameter.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/b2b/add-users-administrator
https://docs.microsoft.com/en-us/powershell/module/azuread/new-azureadmsinvitation?view=azureadps-2.0