My wife and I are in a position where her ESPP shares are finally in a qualifying position and as such I'm planning to sell a good chunk of it to diversity our holdings. Through Vanguard she currently has a Roth IRA and I have a Roth IRA, a traditional IRA from a 401k rollover, and a traditional brokerage that I opened while in college. We already max out the Roth IRAs each year.
My question is when we sell her ESPP shares, does it make more sense to open her an individual brokerage account or open a joint brokerage account and move my current brokerage assets to the joint account?

Open joint.

Just makes things easier if something happens to you or her.

Either that or make sure you ask for "transfer on death" for your individual taxable accounts.

The iras already have bene's

If not opening joint (joint tenant with right of survivorship)

Then single name accounts should have "transfer on death" instructions to make sure things go smoothly.

This depends on your state - Texas is community property so income is automatically joint. Inheritance - no

where did your individual funds come from?

How solid is your marriage HA. Personally I would just put it all in joint. Professionally I usually counsel clients on the pros/cons. For example moving dividends out of individual accounts to joint quarterly is a PIA