1. Where do vendors place their Paypal emailadress for parallel or chained payments?
vendors place their Paypal email address in Seller Dashboard > Settings > Payment Method > paypal

2. What paypal-account does vendors needs? Just a personal account or a business account?
Vendors should have Business account of paypal.

3. If a customer is buying at site, and pay with paypal. What order-status the order get? Does the vendor see if they got payed or if the order is canceled?
If the payment is confirmed then the order status will be completed else pending/on-hold. Vendor or seller can view every status for every order and payment.

4. If I use parallel payment as option, how the checkout looks like? For example Product1 from vendor1, product2 from vendor2 and commission 10% to site owner? Or how it looks like? Any demo please?
The checkout page will be the same for dokan. For paypal payment here is a demo.
paypal-adaptive-payment

5. Does the withdraw-section gets hidden when enable the adaptive-paymnets ad-on?
Yes withdraw-section gets hidden when enable the paypal adaptive paymnets ad-on.

6. If I use chained payments, can I set all fees on vendors and no fee for first receiver?
Yes you can set all fees on vendors and no fee for first receiver for chained payments. Just make sure that vendors use business account and first receiver use personal account of paypal.

paypal adaptive-payments

Some FAQ Regarding Dokan Paypal Adaptive Payments