Because a Saint is not omnipresent, and presumably not capable of hearing contemplative, mental prayer, is it necessary to audibly request saintly intercession? Must one physically ask out loud for a particular Saint to pray for us?
It is true that a saint is not omnipresent. But it is also true that (a) the saint is in eternity, freeing him from the restrictions of time; and (b) the saint is in direct communion with God, who makes it possible for a saint to intercede for us.
All that is necessary is that we ask a saint for his or her prayers. It can be vocally or mentally. God will take care of making sure that the saint hears our prayers.