When you want to move money around, Vanguard just lists all the accounts that are yours or that you have Full Agent Authorization for, they are not offering tax advice.
My understanding of the tax laws are that Roth Conversions from your IRA to spouse's Roth are not allowed and would be an excess contribution to spouse's Roth, that would be subject to 6%/year penalty until withdrawn. It could be fixed by October 15 of the following year without penalty as long as the incorrect amount and any gains on it were withdrawn, if the tax return had already been filed, an amended return would need to be filed.
As an aside, for people that made excess contributions long ago, the SECURE 2.0 passed an update on the statute of limitations. It used to be that the clock started when you filed form 5329, telling them you messed up and how much tax you owed, meaning the statute of limitation clock never actually started, so IRS could go waaaay back at any time. SECURE 2.0 updated the law to start the statute of limitations clock when you file taxes for the year. So folks that made a boo-boo long ago are now off the hook.