1) When someone sells a mutual fund the mutual fund must sell shares of the underlying stocks to raise the money to give the seller their money. Selling stock often results in realizing gains which are then distributed to all remaining shareholders of the mutual fund.
2) Here's a good description of qualified dividends:
https://www.fidelity.com/tax-information/tax-topics/qualified-dividends. Basically, it has to both with how long you have owned the mutual fund, and how long the mutual fund has owned the underlying stock before/after the stock issued a dividend. Qualified dividends and long term capital gains receive preferential tax treatment while non-qualified dividends and short term capital gains are taxed the same as regular income.
3) Yes, you're responsible for paying tax on $34,099.45 ($14,856.14+$19,243.31), but since qualified dividends and long term capital gains are taxed at lower rates (see
https://www.nerdwallet.com/blog/taxes/capital-gains-tax-rates/) you may owe less or no tax on $24,475.77 ($5,232.46+$19,243.31), and you'll owe tax at your normal marginal tax rate on the remaining $9,623.68 ($14,856.14-$5,232.46) non-qualified dividends.
4) Yes, I believe all capital gains and dividends are included in ACA MAGI (among other MAGIs) even if the qualified dividends and long term capital gains are in the 0% tax bracket.
It's usually not recommended to hold funds of funds like VWIAX (or target date funds) in a taxable account because, as you're seeing, they tend not be very tax efficient.
It looks like in 2018 VWIAX had a capital gains distribution of $2.52160 indicating that you have 19,243.31/2.52160 = 7,631.388 shares. I get a slightly different number of shares if I add up last years dividends and divide your dividends by that, but that's probably because you have dividends being reinvested. That just means the 7,631.388 shares is what you had at the end of the year when the capital gains distribution happened, not what you had earlier in the year. I'm too lazy to do the math to figure out what you had at the beginning of the year.
Given yesterday's close of $64.94/share times 7,631.388 shares the current value is $495,582.34 (you probably have a bit more now given dividend reinvestment for 2019). The Vanguard total stock market index closed yesterday at $74.55, so you could instead own $495,582.34/$74.55 = 6,647.65 shares of VTSAX. Last year VTSAX had no capital gains distributions, and dividends of $1.263/share, so you would have had total dividends of $1.263 x 6,647.65 = $8,395.98. This is actually slightly high, because you didn't actually own that many shares for the first three dividend payments (that didn't happen until the end of the year), but it's close enough to see the point. Do you own math. There were lots of numbers flying around there, so I might have messed something up.
Now, obviously you shouldn't go from VWIAX with its 60% bonds to 100% stocks, but that difference in "forced" realized income from the two funds shows why the usual recommendation is to hold bond funds in tax advantaged accounts and stock funds in taxable accounts. You can learn more about that here:
https://www.bogleheads.org/wiki/Tax-efficient_fund_placement