I can confirm that custom skins packed into VPK addons do not work in any game mode aside from Campaign. I tested every mode the day addons were instated and have continued to test after every update since then. Just to double check, I tested a custom skin addon moments ago and can verify that nothing has changed. They still do not work in Versus, Realism, Survival, or Scavenge.
I do not feel that alienating four of the five available game modes is worth the trouble or headaches that a VPK addon would offer. Don't get me wrong; I despise the manual installation method just as much as the next guy. I even have VPK versions of my skins ready to release in a heartbeat. However, in their current state, they're virtually broken, and a non-working alternative is not an alternative for me.
If you would really like, I can host my VPK versions of my skins and send you a link, but I stress the fact that they simply will not work outside of Campaign.
In regards to your message: Custom skins packed in VPK addons will only take effect in Campaign mode. They will not work in Versus, Realism, Survival, or Scavenge. This issue leaves the addons quite limited in use, and I would rather not update my releases to VPK files because of it.