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