down about 50% on my daily XVG earnings too...
Again, this is because the difficulty has gone up because more people mine verge. If your hashrate stays the same, earnings will drop.
But you can't just blame difficulty on this. Even if you go whattomine, it will say you should be getting 37 xvg coins dual-mining with eth @ 12.1 Gh/s hashrate, but in reality I'd be lucky to get 30 coins a day. It is usually 24-26 xvg a day which is about 35% less of what i should be getting.
Whattomine you can use as estimate and gives you a theoreticcal figure.
In practice you should consider orphans, your hashrate ( not every card is optimised 100% to 31.3 mh ) , luck ( one day you find more then another day ) and the difficulty spikes. Difficulty spikes occur when pools like zpool.ca switch from coin A to B when mining that is more or less profitable ... so difficulty changes per second / minute / hour where whattomine only provides you with a snapshot/moment.
I checked my pool and it performs (near) optimal. There are no things i can come up with to improve and the pool is rock stable.