Hi Minkwan -
The login REST API is generally only useful for non-interactive work - you login "via code" and then "run other code" under that identity.
You want to use Trusted Authentication for the work you're mentioning.
You can read about it here:
We used SAML SSO for Tableau and it does support embedded view feature w/o second login. When you embed Tableau views to your web, although SSO works, one thing to be aware is that the web user has to be a valid Tableau server user. This can be difficult for some new users who have web access but have not got Tableau access yet, then those new users will not be able to see the embedded views.