Legally, there is no minimum amount needed to open a Roth account. Some specific investments held within a Roth might well have minimums. Make sure you are aware of the differences. I am not a Vanguard client, so I'm not saying anything specific about them.
I would suggest questioning other large brokers on that specific question; "can I open a Roth IRA with a $1 deposit?"
In my view, a Roth account is such a valuable structure that even if opened for just $1 invested in a MM fund, it starts the five year clock ticking, and can later be used to convert tIRA funds into even if you later have no earned income to open a Roth account with.
Being in a low tax bracket now makes it an ideal time to plow as much money into a Roth as you can bear to save.