So it's a matter of not knowing what you don't know, and you gain knowledge by doing deals, asking questions, as you're doing here.

Now, my Vanguard brokerage fund could lose money (December 2018), or I can make gains (up 14% in 2019).

Your income will be on the capital gains and (assuming they were rental properties for the past 4 years) the recapture of the depreciation expenses allowed or allowable when the properties were rented.You can't avoid the tax by re-investing unless you establish a 1031 exchange prior to the sale.

REITs1) act like a mutual fund so liquid2) ordinary income so taxed at your tax rate3) returns are unpredictable4) intangible assetReal Estate1) less liquid (you'd want to hold for at least a year or more)2) returns can be projected based on the Actuals used as an input3) long term hold offers you sometimes losses in the first few years via depreciation deduction and then long term cap gains taxed at a max of 20%4) actual asset

I realize I will have to pay gains on it if I don’t do a 1031 exchange in so much time, but I thought if I do flips, we have to always churn the money anyway so why not pay the gains and pay down debt quickly?

Also check with a CPA, I'm not sure you're allowed to depreciate a property that isn't rented, maybe they have a work around.So you're comparing only projected capital gain on a property (including it's holding costs for utilities, insurance, taxes and repairs) vs the state fund. 

You can receive some rental income and then also gain experience as a landlord. 

I wasn’t considering the extra equity that I gained in my calculations and I was counting my mortgage being paid by the tenant adding to my return over time.

In exchange for paying back the taxes, my partner and I would gain 60% equity of the house and resell it (free and clear).

If the numbers work and you have sound exit strategies for the deals you analyze you will be fine. 100% agree with @Michael Ablan in that you likely wont make much on appreciation but you can easily cashflow and gain equity through debt paydown.Closing on a Triplex in the area here in a few weeks.