You want to have an answer for "what's the difference between these two funds" if the IRS asks. VTI holds ~3600 stocks where VOO holds ~500. Unless you're looking at more than 98% overlap in assets, I don't think it's a concern.
I view it as two conditions:
1. share price is below purchase price (taxable loss)
2. avoid doing a buy and a sell of the same asset within 30 days of one another
So you need to look at your transaction history for VTI and VOO. If you want to realize a loss from VOO, check for purchases in the past 30 days.. and stop if you have any. Then for VTI, did you sell it in the past 30 days? If so, you can't move money from VOO to VTI without the wash sale problem.
And then you sell VOO, which generates a "pending credit" in your Vanguard account. When the transaction clears in 3 days, it settles and you get the money. But Vanguard lets you take that "pending credit" and buy shares, like VTI. So you can use up the pending credit and purchase VTI with it.
Then you need a warning for those two holdings: you can't buy VOO for 30 days, and you can't sell VTI for 30 days. You can't reverse what you did for 30 days.
Note if you automatically reinvest dividends, that's two events for the IRS. The fund paid you qualified dividends... and you used the money to buy shares. If that "automatic" action triggers in the same account you sold shares, your "automatic reinvestment" triggers a wash sale. So you might want to avoid doing it right now, near the end of June, if you have automatic reinvestment setup for VOO (for this example of selling VOO to buy VTI).