A vanilla call option has Vega — but it is not pure volatility exposure. The Vega depends on the underlying's price (vega is highest at-the-money and falls as the option moves OTM or ITM), on the time to expiration (vega scales as √T), and on the volatility level itself. A trader who wants to bet that 30-day SPX realized volatility will exceed the level priced into the SPX option chain has a problem: any vanilla long-vol position they assemble will have Vega that decays as the underlying drifts away from the at-the-money strike, and they will need to continuously rebalance to stay in the highest-Vega zone.

The variance swap solves the problem by financial engineering. It pays its holder the difference between the realized variance of the underlying over the contract period and a strike variance agreed at inception — without requiring continuous hedging, without the Vega-decays-away-from-ATM problem, and with a payoff that scales linearly with realized variance.