Here is what you need to do:

Look up vanguard's etf expense ration on their website. Then ask M1 what is the expense ration for that vanguard etf. 9/10 times it will be the same! Unless for some miracle vanguard would give preferential treatment to their own clients and charge them lesser expense ratio than what's stated on their website.

Standard M1 account doesn't charge for trading fees or management fees. If you have M1 plus then you are looking at $125 a year fee from M1.

In my opinion rep from Vanguard is just trying to keep your funds from leaving their platform.

You should dig into this yourself and don't necessarily trust rep from vanguard or strangers on the internet.