Your money was not going to be forfeited to the government. Every state has an unclaimed property division, and rules about how long accounts can be inactive before financial institutions have to turn assets over to the state. You stop getting any interest on the funds, but you can easily claim the money back in most states -- just fill out an online form and submit proof that you are who you say you are.
https://www.missingmoney.com/en/I've not found anything for myself, but I've found hundreds for family members and one multi thousand account for a friend that her dad had forgotten about. He'd died several years back so that was a bittersweet one.
And yes, not anything to be concerned about regarding Vanguard. They were doing what they could to try to get your money reconnected to you and legally that is exactly what they should do (after years/good faith attempts to contact the account holder then turn account/money over to a government oversight to hold until claimed). If I'm reading this right, you never opened any mail/logged into your account over a multi year time? So they had no way of knowing you still live at your address or even on the right side of the sod if you never responded or logged in.
Unclaimed money from deposits, refunds, overpayments, legal settlements, rebates, bank and financial accounts and more get forgotten allllll the time, so there's a place in each state that the funds are sent to be held (over decades even!) until the person or their heirs discover them and lay the claim.
It is always an individual's responsibility to keep track of what accounts are where and update things periodically or at least check every year or so if things seem okay. But lots of people forget, or die, or didn't even realize they were owed anything. You would have had an extra hoop to jump through to get the money back, but they weren't going to steal or misappropriate the money.