# Variance swap replication and variance vega

Noob here.

I've been trying to gain a better understanding of variance swaps and what better way than to replicate it with a portfolio of better understood instruments.

I have read the GS 1999 paper and JPM 2005 paper and think I get how the replication works.

With the assumption of no jumps and full continuous strikes, the replication using options is exact, so the variance swap and the ideal replication portfolio should be indistinguishable. Now we know that the variance swap's variance Vega (d price/d variance) at any given time is simply the variance notional regardless of spot and vol level. So it follows that if I sum (do an integral across strikes) of the options' variance vegas, I should get a constant as well. However, in BS, Vega, gamma, variance Vega are all functions of vol. And vol is NOT a constant function of strike. Does this not mean that a different vol curve would generate a different Vega variance curve? Something is obviously wrong with my argument, but where is it wrong?

Thanks!

• I guess another way to ask the question is, given a volatility-spot curve, for any spot value S, if I sum the vega or variance vega of each one of the options in the replication portfolio using Black Scholes formula, I don't think I'll get the same value for all S values? – RAY Feb 15 '15 at 16:09
• You may be confused by the fact that the vega as you used it only applies in the BS model whereas your portfolio valuation is not based on a model but on market values. When considering a non parametrical skewed implied volatility the notion of vega is not trivial (no parameter to derive w.r.t.). The naive way to do it is to consider a parallel shift of the implied volatility slice at the maturity of the variance swap. This is however not arbitrage free and not very meaningful so we use instead good parametrical functions and derive the shift by bumping one or more parameter of the curve. – vanna Feb 22 '15 at 0:18

• @RAY: When you say, the variance Vega is equal to the variance notional, it is because you derive it from Price = VarNotional * (Variance - Strike). However the Variance in the formula is an estimation of what the realized variance will be at the end of the swap. Whether you estimate this variance with option implied volatilities is up to you. – d--b Feb 15 '15 at 17:25