Why Delta Trades Can Still Lose Vega
Summary
The discussion explains why a strategy that anticipates underlying price moves and seeks delta exposure may still lose money through vega. For European vanilla calls and puts with the same strike, put-call parity implies equal vega; holding equal and opposite positions therefore offsets vega. This result depends on the options having matching strikes and quantities, and does not establish neutrality for positions across different strikes or for a broader trading process.
The second answer points to adverse selection when the trader chooses options to obtain a directional position but does not forecast implied volatility. If options appear expensive under the trader's model, the choice of calls or puts can create an unintended short-vega bias. Later buying options to offset that exposure may happen at unfavorable volatility levels, producing negative vega P&L on average in the described scenario. The question gives no details of the strategy or data, so the explanation is conditional; diagnosing an actual loss would require the option-selection rules and trade history.
Key ideas
- Equal and opposite positions in same-strike European calls and puts are vega neutral under put-call parity.
- Different strikes, quantities, or option-selection rules can leave residual vega exposure.
- Directional option trading without an implied-volatility view can create adverse selection.
- An offsetting trade may be costly if it is added after the initial volatility exposure has become unfavorable.
- The explanation depends on the strategy details and does not diagnose a specific P&L record.
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Full text
# If you try to capture short term delta by anticipating moves in the underlying, why would vega pnl be so bad? # If you try to capture short term delta by anticipating moves in the underlying, why would vega pnl be so bad? Since calls and puts have opposite sign delta, but both are positive vega, it feels like a strategy that buys/sell or sells/buys calls and puts on underlying moves to capture delta should generally tend to be vega neutral. Yet I always find that instead vega pnl always looks very negative. Why could that be, intuitively? ## Answer by LocalVolatility (score 2) https://quant.stackexchange.com/a/29989 I assume you refer to European plain vanilla calls and puts. In this case it immediately follows from the put/call parity in a model-free setting that a call and put option with the same strike have the same vega. Unless two different strikes are involved the resulting portfolio of a long position in one and a short position in the other with the same absolute number of options will be vega neutral. ## Answer by VommaNeutral (score 2) https://quant.stackexchange.com/a/30865 LocalVolatility's answer seems correct in the case where you trade calls and puts of the same strike, in exactly the same proportion, before anticipated moves (in which case, I would ask why not just trade the stock rather than the options?) If that's not the case, I can't presume to know exactly what your strategy is; are you 'anticipating' an underlying move, say a move up, and then choosing options that look 'cheap' (relative to some pricing model) to trade into positive deltas? Is there a systematic way you're choosing the option to trade, or is it more ad hoc? In either case, if you're not anticipating changes in implied vol then adverse selection is probably going to get the better of you. For example, if you anticipate that the underlying will go up, and the market is trading at a high implied volatility relative to what you're pricing in (with no real signs it's going to come down), option prices will look higher across the board. In this case, since you want to accrue long deltas, your pricing (or intuition) will probably tell you to sell puts since they're looking expensive right now rather than buy a call which doesn't look too attractive if you're not pricing in volatility. In this case, you will naturally lean towards doing more selling than buying - if you then decide you don't want the vega risk and buy the corresponding call later, you're at a disadvantage - you essentially got into a vega position at a random price, and so you're naturally going to (on average) buy it back at a bad time. It stands to reason that vega PL would be quite negative in this scenario.
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