I'm afraid you can't. Even if you buy NewscasterVocalizer PRO 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 PRO is an Upgrade or Up-sell which adds the PRO features in your NewscasterVocalizer account.
3. Since NewscasterVocalizer PRO 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 PRO in your account.