Ok, I think I've got this, here's the results on silo 56:

The following packages will be upgraded:
  libaccounts-qt5-1 libonline-accounts-client1 libsignon-extension1
  libsignon-plugins-common1 libsignon-qt5-1 qml-module-ubuntu-onlineaccounts
  qml-module-ubuntu-onlineaccounts-client qtdeclarative5-accounts-plugin
  qtdeclarative5-online-accounts-client0.1 signon-plugin-oauth2
  signon-plugin-password signond
12 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 407 kB of archives.
After this operation, 2,048 B of additional disk space will be used.
Get:1 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main libaccounts-qt5-1 armhf 1.14+15.04.20150930-0ubuntu1 [28.7 kB]
Get:2 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main libonline-accounts-client1 armhf 0.7+15.04.20151009.2-0ubuntu1 [13.7 kB]
Get:3 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main libsignon-extension1 armhf 8.58+15.04.20151001-0ubuntu1 [18.8 kB]
Get:4 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main libsignon-plugins-common1 armhf 8.58+15.04.20151001-0ubuntu1 [16.2 kB]
Get:5 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main libsignon-qt5-1 armhf 8.58+15.04.20151001-0ubuntu1 [54.3 kB]
Get:6 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main qml-module-ubuntu-onlineaccounts armhf 0.6+15.04.20151007-0ubuntu1 [47.7 kB]
Get:7 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main qml-module-ubuntu-onlineaccounts-client armhf 0.7+15.04.20151009.2-0ubuntu1 [6,988 B]
Get:8 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main qtdeclarative5-accounts-plugin armhf 0.6+15.04.20151007-0ubuntu1 [1,390 B]
Get:9 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main qtdeclarative5-online-accounts-client0.1 armhf 0.7+15.04.20151009.2-0ubuntu1 [1,462 B]
Get:10 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main signon-plugin-oauth2 armhf 0.23+15.04.20151001.1-0ubuntu1 [52.4 kB]
Get:11 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main signond armhf 8.58+15.04.20151001-0ubuntu1 [153 kB]
Get:12 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main signon-plugin-password armhf 8.58+15.04.20151001-0ubuntu1 [11.9 kB]
Fetched 407 kB in 4s (99.6 kB/s)              


So it's only installed packages from silo 56, nothing from vivid and nothing from stable snapshot ppa.

$ adb shell apt-cache policy signond
signond:
  Installed: 8.58+15.04.20151001-0ubuntu1
  Candidate: 8.58+15.04.20151001-0ubuntu1
  Version table:
 *** 8.58+15.04.20151001-0ubuntu1 0
       1100 http://ppa.launchpad.net/ci-train-ppa-service/landing-056/ubuntu/ vivid/main armhf Packages
        100 /var/lib/dpkg/status
     8.57+15.04.20150616-0ubuntu1 0
         50 http://ppa.launchpad.net/ci-train-ppa-service/stable-snapshot/ubuntu/ vivid/main armhf Packages
     8.57+15.04.20150204.1-0ubuntu1 0
         50 http://ports.ubuntu.com/ubuntu-ports/ vivid/main armhf Packages

this shows the correct silo pinned at 1100, and the stable ppa & vivid itself pinned at 50, which is beneath locally installed packages, so they won't be installed unless a dependency specifically brings them in.