Brokers monitor all of the trades at all times, not just for the first 6 months. This monitoring relates to meeting the opening margin and subsequent margin maintenance requirements.
Reg T sets forth minimum margin requirement levels. Brokers can require more restrictive amounts. Brokers can also restrict what level of risk they can provide customers as well as what securities they allow customers to trade.
Vanguard wants to dissuade trading. That is evidenced by their commission schedule. For example, if account size is less than $50k, the fee is $7 for the first 25 trades and $25 per trade thereafter. They are doing the same with short option writing, namely forcing your to call them to do so. It's all perfectly legal. They are under no obligation to let you do whatever you want to do.
As for the idea of frontrunning, the first paragraph in your link answers that:
Front-running is when a broker or other entity enters into a trade because they have foreknowledge of a big non-publicized transaction that will influence the price of the asset, resulting in a likely financial gain for the broker.
I doubt that your trades are large enough to move the market. Vanguard isn't an investment bank with a trading desk so I doubt that they have any interest in front running your small trades (I assume that they're small because if otherwise, you'd be trading elsewhere). Traders are not their target audience.