You can specify which lots to sell.

I've never sold my RSUs immediately, but I have sold to diversify, and I have told the brokerage exactly which lots (vestings) I'm selling.

Just as a test, I put in a sell order (I'm previewing it now), and I've told it to sell 1 share from each of 12 different lots. 12 total shares. But that's the software provided by the brokerage my company has selected. Yours may vary.

Time to go hit cancel.