In addition, if you are going for spell penetration cap, the cap is 228. There is an enchant that will get you 70 spell penetration for your cloak. There is also a PvP cloak, Vicious Gladiator's Drape of Diffusion that has 201 spell penetration. It makes it look like you could hit the cap with just these two items, but you don't want to sacrifice hit for spell penetration, which depending on your gear, you might be doing.
Moonkins with Other Offspec - Looking at the moonkin's vicious 5 pieces for PvP, this is the slot breakdown.
1 Meta Socket - Meta Gem of Your Choice (See prior blogs for my PvP choice)
3 Red Sockets - Use Brilliant Inferno Ruby
2 Blue Sockets - Usually Purified Demonseyes
2 Yellow Sockets - Usually Reckless Ember Topazs
If using PvE gemming strategy, you would end up with + 200 Intellect, + 40 Spirit, + 40 Haste. If you take out all the blue and yellow sockets and replace them with Vivid Dream Emerald, you give up 40 Spirit and 40 Haste for + 80 Spell Penetration. Even with the enchant on a cloak, you still wouldn't hit the spell penetration cap. To hit the cap, you would also have gem with the Vivid Dream Emerald in your pristmatic slot in your relic, in your belt buckle slot, and in 2 red sockets, which means now you are giving up 80 intellect (assuming you would have used Brilliant Inferno Rubies for all those slots) as well to hit the spell penetration cap.
All this math comes down to a single fact. If you are just building a moonkin PvP set, you will need to get the Vicious Gladiator's Cloak of Diffusion. Once you do that, you won't need to gem with the Vivid anyway. If you need the Vicious Gladiator's Cloak of Meditation for being hit capped (since it has Spirit on it), you will need to gem with the Vivid Dream Emerald to hit the spell penetration cap, but be prepared to give up a lot of helpful DPS stats to get it.
Moonkins with Resto Offspec - First thing that is necessary is knowing spell penetration does nothing for heals at all. It has no benefit. A few blog post ago, I mentioned that you can switch out 2 of our moonkin 5 piece to get the resilience bonus for your restoration spec and pick up more spirit. The two pieces in particular were the shoulders and the chest. The two blue gem sockets and one of the yellow gem sockets are still around. So now, you have 60 spell penetration in a restoraton set that doesn't matter about it.
If you are a moonkin with a restoration offspec, you should never gem spell penetration in your gear, which means you can forget about the Vivid Dream Emerald. You can save up your Conquest points and get the Vicious Gladiator's Cloak of Diffusion to use just for moonkin alone and get the spell penetration enchant. But have it as a seperate equipment set, not as a part of the shared set.
So all in all, the biggest advice I could give is go for the Vicious Gladiator's Cloak of Diffusion and forget about using Vivid Dream Emeralds entirely. However, remember, don't sacrifice hit for spell penetration. It is useful, but not more useful than hit.