Upgrade your vanity armor with particle effects....

[Energizeric]

I like the particle effect idea, but I don't think the +10 primary stat should be added. Also, I think if people have two slots ( one on each item ) then over lapping effects won't work well.
Personally I think there should be just the one slot on the plate/coat/armor piece.

Yes, only on the armor. No helm upgrades.

Regarding the stat bonus, I figured it would make it more interesting and desirable.

In Pocket Legends we had dragon vanity sets which gave stat bonuses. Eventually it was decided this was a bad idea because after some time everyone was wearing the same exact vanities and we all started to look alike. But in this case, you can use the particle gem on any vanity of your choice, so we will all still be able to have our customizable looks. So I don't see this being an issue.