Is it a good time to buy TVIX stock?

No. I lost quite a bit on TVIX even when I was bearish on the market like 3 years ago. One I was wrong. Also it has nearly doubled in a week, the markets are stressed out, but any sign of good news and TVIX will plummet. Further you should understand how an Exchange traded note works. It is not guaranteed to perform in line with the VIX and is subject to supply and demand aspects since they cannot issue new shares unless approved. For this reason they can be traded at a premium or discount to their market value.

If you are long equities, my thought is why not take 20 to 25 percent of your portfolio and buy TVIX as a hedge against a market downturn?

It isn't a stock, it's basically an ETF that bets on the volatility of the stock market. It's no different than going to the casino. You are speculating on the behavior of investors, not investing in a business. Personally I think this is an example of a useless financial instrument that contributes nothing to the economy, except to give saps a chance to gamble without going to a casino.