I am looking to do the same thing. I also use Vanguard as my IRA / Roth IRA custodian.
I was hopeful that it'd be possible to complete this request electronically, but the literature I found on the Vanguard website makes it look like I need to fill out a 4-page form "Authorization to Recharacterize IRA Assets" and mail it in. Is that the same thing you're seeing?