No, the most basic and notable difference is that the VTI is an ETF. At the same time, the VTSAX is a mutual fund, meaning that you would need a minimum of $3,000 to start investing there.
VTI isn’t more tax-efficient than VTSAX. Vanguard has its own patented way of avoiding taxes on VTSAX, which means they both get treated equally when it comes to taxes.
Yes, you can, as most shares that operate based on ETF shares allow you to convert from conventional shares to ETF shares, especially if the providers are the same business.
Yes, both of these funds are identical in the long run and pay off dividends regularly, either quarterly or yearly. You can also decide whether you want to reinvest your dividend or have it paid out to you.