I can't think of any reason to not move to Vanguard. Your portfolios sound pretty simple. If Betterment is just investing in index funds on your behalf, just cut out the middle man and enjoy your extra $1800 a year.
I think brokerage firms try to take advantage of complexity bias. Most people assume a more complicated solution is a better solution. That's not true. You don't need someone or an algorithm constantly pulling levers and turning dials on your portfolio to get the best returns. A lot of people here have become wealthy just throwing money at total stock market, S&P 500, and bond funds at regular intervals and rebalancing without the aid of an advisor.