Let's say your employer plan is terrible, with a 2.00% expense ratio and 25% employer match. If you put $4,000 you get a total of $5,000. That extra $1,000 of free money can pay that expense ratio for a very long time. If you have unpaid credit card debt, invest in the 401k to the point of the match, then pay the credit card debt - it's that good.
Vanguard is a company that structures investment funds. They have customers like you, individuals, and institutions (company 401k plan, for example). When you send Vanguard a check, that money starts in a money market fund. It's cash, and not earning much. I take it you've already done this to get your Roth account setup, and then purchased VTSAX with that money.
When you create an individual account, you need to fit your investment to the fund minimums:
$113 ($7 commission + 1 share of VTI) to $2,999: need to buy an ETF, like VTI
$3,000 to $9,999: can buy "investor" class of fund, like VTSMX
$10,000 to millions: can purchase "admiral" class of fund, like VTSAX
(millions lets you buy various institutional classes of funds)