That's part of the surprise - I can't speak to the specifics for US citizens more than others in this post have as I'm not based there. Tax definitely wasn't determined _at time of vest_ for anybody though, it was time of liquidity.
In Australia we were granted options, which ordinarily are taxed at time of exercise. Lots of people were surprised to discover, almost a full year after the IPO, that those options were also subject to a tax deferral scheme and any tax already paid at exercise wasn't sufficient. The actual taxable amount determined by HashiCorp and the ATO was the $80 IPO price. If you sold the full amount you were entitled to (10% of your vested holdings) at the IPO you were probably fine. If you sold nothing, because you thought you had already paid the required taxes, by the time you received the tax statement the value of your stock would have been less than what you owed in taxes.
In Australia we were granted options, which ordinarily are taxed at time of exercise. Lots of people were surprised to discover, almost a full year after the IPO, that those options were also subject to a tax deferral scheme and any tax already paid at exercise wasn't sufficient. The actual taxable amount determined by HashiCorp and the ATO was the $80 IPO price. If you sold the full amount you were entitled to (10% of your vested holdings) at the IPO you were probably fine. If you sold nothing, because you thought you had already paid the required taxes, by the time you received the tax statement the value of your stock would have been less than what you owed in taxes.