I'm afraid you can't. Even if you buy NewscasterVocalizer Agency without the Standard Version and even if you buy without the Standard Version you will not be able to login.
Here is why:
1. NewscasterVocalizer Standard is the main product which is either an Yearly Subscription or Monthly Subscription.
2. NewscasterVocalizer Agency is an Upgrade or Up-sell which adds the Agency features in your NewscasterVocalizer account.
3. Since NewscasterVocalizer Agency is not independent, you will need to subscribe for the main product NewscasterVocalizer yearly or monthly subscription first. Else you can't login with only NewscasterVocalizer Agency in your account.