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