Yes, the cap goes back on after filling. Also, note that every now and then, the Sapphire seems to produce very little vapor, even though it's got lots remaining, the battery is good, and the draw isn't clogged. You can fix this by popping off the mouthpiece, removing the blue plug, and then putting the blue plug back in (as well as the mouthpiece, of course). This will "fix" the vacuum lock that's preventing the PG from wicking properly. It doesn't happen often, but when it does, you'll know what to do.
I used a Dremel to grind down the mouthpiece end of the eGo cone a little. I'm now using the actual "Bud" manufactured cone for the Sapphire, which adobeWan got for me from a friend of his in Australia. (It seems the only place any of us could find that sold them was in Australia, and the place didn't ship out of the country. It was a wonderful fluke, and fuck combustion's internationally connected membership, that let it happen).
Pure Gold mixes pretty well with Propylene Glycol (and Vegetable Glycerin), if you're looking to mix some zero nic e-juice in for the flavor. I would suggest starting with very little e-juice in the mix (i.e., mostly Pure Gold), and use an e-juice base of anywhere between 100% and 75% Propylene Glycol (meaning anywhere between 0% and 25% Vegetable Glycerin) as a start. Any more Vegetable Glycerin and the mix will be a bit thicker than you really want. Shake well.