I’m a VSF supporter and our website works well online.But now it is only integrated with paypal, we need more payment methods.Because we wanted to push the VSF to 10 or more countries, but the payment method got in the way.
I’ve implemented many payment methods in VSF1. Currently, for card payments I recommend Stripe. This is my enhanced version that supports 3DS Auth (However, I have to update readme for details about 3ds) also: https://github.com/new-fantastic/vsf-payment-stripe
It works pretty well with Magento 2.
Another payment I could recommend you is Braintree, it was not that hard to implement PayPal & Card with Braintree: https://github.com/danrcoull/vsf-payment-braintree
In the following months, maybe there will be a stable solution for payments with Adyen but I do not recommend the current solution.
Thanks for sharing this. Has anyone here experience with integrating Adyen into VSF? But I guess it should be similar to the above examples.
I implemented Adyen with M2 backend. However, it required so much effort from me and help from Magento’s Developer. I will try to prepare Open Source module for Adyen that supports 3ds2 & 3ds1 in the following months. The current shared solution does not offer so much 
I want to set up the German station now, but I can’t solve their payment method.I see other German websites have their payment method, but I don’t know how they do it
That sounds great ！ 
To be honest, I also faced problem with slow Magento2 API Cart Interface but did not really solve it. You should probably ask your Magento’s developers for it.
Kind of workaround is just calculating prices client’s side while user is waiting for the response - then smoothly update it.
Thanks, * I’m the Magento developer! * But There’s nothing I can do about it, right