Hey @Markus,
I'm happy to help! If you're looking for direct access to VisaNet, then take a look at the Visa Payments Processing APIs. Links are given below.
VPP Overview - https://developer.visa.com/capabilities/vpp
VPP Documentation - https://developer.visa.com/capabilities/vpp/docs-getting-started
For Visa Direct, take a look at this documentation link. https://developer.visa.com/capabilities/visa_direct/docs
In particular, see the top few paragraphs, in the grey box with the exclamation mark, that talks about the requirements to use Visa Direct.
If you meet the Visa Direct requirements, then fill out the Visa Direct Program Implementation Questionnaire and submit it to the email address listed on that form. There's a link to the questionnaire within the grey box that discuss the requirements.
Hope that helps!