I'm afraid you can't. Even if you buy Vidently Agency without the Standard Version, you will not be able to login.
Here is why:
1. Vidently Standard is the main product which comes as a Vidently Personal or a Vidently Commercial License.
2. VidVamp Agency is an Upgrade or Up-sell which adds the Agency features for your Vidently Software.
3. Since Vidently Agency is not independent, you will need to subscribe for the main product a Vidently Personal or a Vidently Commercial License first. Else you can't use the software by only having Vidently Agency in your account.