I admire MTE's patience in this thread. I will attempt an explanation for candeo: If frontmonth and backmonth are both trading 15% volatility and you do a 1 by 1 spread (long front/short back), you will be net short vega. But this net short position results from being LONG vega in the front month and short a larger amount of vega in the backmonth. Now if vol in both months goes down to 14%, then you should make money. But if frontmonth vol gets crushed and backmonth vol stays the same or goes up, you will get killed on your position, as you are losing money on both sides of the trade. Vega tells you how much money you will make or lose given a 1% absolute move in volatility. So if you are long 40 vega in frontmonth and short 100 vega in back month, your simulator will tell you that you are net short 60 vega. This number masks your calendar risk, for there are numerous scenarios where the vol comes in and you lose money. For instance, in the above example, if frontmonth vol goes from 15% to 10%, you will lose $200 on that leg of the trade. If the backmonth vol comes in from 15% to 14%, you will only make $100, so you will lose $100 on net, even though the vol came in across the board and you were short vega. I hope this clarifies matters some.