You pay taxes on RSUs not at vesting, but when they settle into shares. This might be shortly after vesting; it might be delayed until an acquisition/IPO. (generally it is delayed for companies far away from IPO).

The delay causes multi-year income to be batched into a single year. With a progressive tax system, that results in your money being taxed at a rather high marginal tax rate: If you have a substantial amount you vest a year, it's easiest to use the highest marginal bracket as a conservative guess of what you'll be taking home. In California, that's somewhere on the order of 48% combined state + federal.