^GSPC is an index, not an actual instrument that you can trade. It ignores dividends.
So, it is not the same as long and short the 500 stocks, in such case, you may gain dividends or pay margin interests and pay dividends.
Vanguard 500 Index Fund Investor Shares is a mutual fund that tracks S%P 500. It pays dividends and incurs expenses. It also holds a small % of cash and bonds.
To track the performance of actually buying/selling SPX stocks, I recommend using VFINX as a proxy.