Hennie Deurhof
when you install 10.6 or above, the default print service that comes with the server install is capable of printing vector-tiles-layers at higher quality - the downside of using the default print service is that it comes with only 8 layout templates and you can't use your own layout templates.
Custom print service, with your own layout templates, can only be published to an on-premises server. When you publish/share a gp tool (as you know print service technically is a gp service) from ArcGIS Pro, as of now, it requires the server being federated to an on-premises portal.
For high quality vector-tiles-layer printing from agol or any webapp, you need to use a a print service that is published from ArcGIS Pro 2.1 or above