Does order(c.security, -c.order_size) account for transaction costs, borrowing costs and vwapping using a symbol specific profile based on volume and volatility? Probably not a big deal for SPY, but would make more of a difference for a larger portfolio.
You can specify a commission model (https://www.quantopian.com/help#ide-commission) and a slippage model (https://www.quantopian.com/help#ide-slippage).
Right now, you have to enter parameters into one of our pre-made models, but soon (next week probably) you'll be able to write a custom model that uses whatever inputs you'd like to calculate commission and slippage.
What does it mean to account for "vwapping"?
I mean I know what VWAP is, I write trading algos for a living but I have no idea what your trying to ask here:)
The more active (frequency and volume) your backtest is, the more inaccurate the results will be if you just use the closing price.