Ginny Marvin, now of Google, said on Twitter that Google considers subdomains, country TLDs and vanity URLs owned by the same company would be considered a first party context. One caveat is that Google has a 5 domain limit per first-party set for vanity URLs.

Navah Hopkins asked the question on Twitter:

Ginny came back and explained that Google considers subdomains, country TLDs and vanity URLs owned by the same company would be considered a first party context. With that caveat that Google has a 5 domain limit per first-party set for vanity URLs.

Forum discussion at Twitter.