I used a visitor id in the page tracking. So when a user logs in on the subdomain service.test.com I forward the visitor id, when the user switches to test.com I also forward the visitor id. Orders only happen on test.com. The result of the implementation is that order participation for a page or domain from service.test.com that worked before is because of the visitor id not working anymore, so showing zero since the implemenation.
Virtual report suites (which you're using, as mentioned in your comment) derive their data from the "main" report suite. So any data that you track to that main report suite will be available in the virtual report suites too (subject to what you're segmenting out in the virtual report suite's setup).
So there is no need to forward the visitor ID, because all of the virtual report suites will use the same visitor ID that is tracked to the main report suite.