We run a Magento 2.1.8 Shop with Amasty Geo IP Redirect Module. 

Unfortunatly it is not working correctly with varnish. 
Amasty wrote they know no working solution right now.
I looked up for other modules - all not working with Varnish right now.

Has anyone any Idea how to achieve this? 

Our current state is, that the session parameters for the redirect module are not written and the user gets redirected everytime. So switching the store is not possible.

This is a serious problem for us. Disabling Varnish is no solution since the performance increase is neccessary.