3. Transaction costs: If you are trading Vanguard ETFs at Vanguard the transaction fees are $0.
Fixed! Actually since August 2018, Vanguard charges $0/trade for ~1800 ETFs, including iShares, SPDR, Schwab, ...
Your original statement is still correct, but I like to get the word out whenever it comes up. It's especially relevant to tax loss harvesting, since you can switch out of Vanguard ETFs and into Schwab/iShares.
Back to OP's assumption that it's too late.. why? Monday is the last trading day of the year, and anything you sell on that day applies to your 2018 tax bill. If you have something at a taxable loss, you can sell and realize that loss.
Without transaction fees, I don't see the downside in tax loss harvesting. The IRS subtracts money from what you owe on taxes, and you keep that money invested. Rather like an interest free loan for as long as you like.