I was suggesting using VIX and SP500 (and similar VIX like index) to backtest, and not to worry about strike specific vol. Of course this is just an approximation.Why do you think variance swap is a better proxy for checking volatility risk premium? I would say it's not true.
PS. really, equity index variance swap data is available for free? Unless you are talking about the few VIX-like indices out there, it's hard to find reliable variance swap data even if you are willing to pay for it.