I don't think I mentioned before rubric #2: The procession may begin from outside the church building, from the narthex, of from the back of the nave. (Altar Book, page 501) Hand in hand with the above rubric is this one (p. 502): If the procession begins outside the nave, the congregation gathers around the processional cross.
Worshipping as we do right next to a loud and busy highway does not lend itself practically to beginning outdoors, either for Vigil or for Palm Sunday. Don't ask me how I know this! But if you have a quieter setting AND the weather is cooperating, there is something absolutely wonderful about having the people gather outside for the initial liturgy on Palm Sunday and then entering the Church building singing "All glory, laud, and honor!" as they wave their palms in honor of the King.