There's often confusion about which one to use. It all hinges on where your money's coming from.
Initiate an account transfer to move money from an IRA or other account held at another company into a new or existing IRA or other Vanguard account.
Transfer an existing IRA to Vanguard
Initiate a rollover to move money from an employer-sponsored plan, such as a 401(k) or 403(b), into a new or existing Vanguard IRA®.
Roll over your 401(k) or other employer plan to a Vanguard IRA