1.Related to your first question:
In the GCP and EU DATA PROTECTION DIRECTIVE document it states:
In cases where no region is selected, or where a particular service does not support a particular location, Google will store and process data in other locations where it (or its agents) maintains or operates facilities. Data location options are clearly identified in the Google Cloud console
In the Google Cloud Vision API you don't have the option to choose location, so you cannot tell for sure that all the servers involved are going to be located in the EU.
I opened a google cloud public feature request that you can follow for updates. You can click on the star icon so that you receive email updates about this.
2.Second question:
If you are only using Google Vision API you will only be charged for its use. Here you have the official docs for GCP Vision API pricing