The preludes usually begin about ten minutes before the ceremony is scheduled to start. It is pleasant to have music in the background as people are entering the church, finding their seats, pinning their corsages & loading film in their cameras. I suggest two or three "instrumental" selections followed by one vocal selection. You should speak to your musicians about doing a couple "low-key" tunes at that time. About three minutes before the ceremony is to begin, the vocal prelude should be sung.... that way, the congregation knows the show is about to start!
When choosing a prelude song, keep in mind that it should set the tone for the wedding mass - it should reflect the sentiment of the day: love. The prelude is an excellent opportunity for a more secular song - if you have one in mind which strikes your fancy, but isn't too appropriate for, say, communion.
Some PRELUDE suggestions:

AVE MARIA by JS Bach / arr. Gounod
This traditional hymn is the "Hail Mary" prayer sung in Latin. Usually this song is sung as the mothers of the bride & groom are being escorted to their seats. Can be accompanied by any instrument, though it does sound its traditional best with the organ.
LYRICS:
Ave Maria, gratia plena, dominus tecum benedicta tu in mulieribus et benedictus fructus ventris tui Jesus. Sancta Maria, Sancta Maria, Maria, ora pro nobis, nobis peccatoribus nunc et in hora in hora mortis nostrae amen.

FROM THIS MOMENT ON by Twain & Lange
A popular song which translates well into a modern-style wedding, I have done this with guitar, piano and even organ music. Piano & guitar work very well with this one, and it is a lovely sentiment... and even has a spiritual feeling for those churches who like their music liturgical!
LYRICS:
From this moment life has begun. From this moment you are the one. Right beside you is where I belong, from this moment, on. From this moment, I have been blessed. I live only for your happiness and for your love, I'd give my last breath, from this moment on. I give my hand to you with all my heart. I can't wait to live my life with you, I can't wait to start. You and I will never be apart. My dreams came true because of you. From this moment, as long as I live, I will love you, I promise you this: there is nothing I wouldn't give from this moment on. You're the reason I believe in love, and you're the answer to my prayers from up above. All we need is just the two of us. My dreams came true, because of you. From this moment, as long as I live, I will love you. I promise you this there is nothing I wouldn't give, from this moment - I will love you, as long as I live, from this moment, on.

GROW OLD ALONG WITH ME by John Lennon
This is a gorgeous song which sounds good with nearly every type of instrument. I love these lyrics, simple and sweet.
LYRICS:
Grow old along with me. The best is yet to be. When our time has come, we will be as one. God Bless our love, God bless our love. Grow old along with me. Whatever fate decrees, we will see it through, for our love is true. God bless our love, God bless our love. Spending our lives together, man and wife, together, world without end, world without end. Grow old along with me, two branches of one tree. Face the setting sun, when the day is done. God bless our love, God bless our love.

HOW CAN I KEEP FROM SINGING?   traditional-style hymn with varying choices of liturgical or non-liturgical lyrics. suitable for any instrument
LYRICS:
My life goes on in endless song, above earth's lamentations. I hear it echo in my soul - how can I keep from singing? No storm can shake my inmost calm, while to that rock I'm clinging. Since love is Lord of heaven & earth, how can i keep from singing?

HOW DEEP IS THE OCEAN? - 1930's ballad by Irving Berlin
LYRICS:
How much do I love you? I'll tell you no lie... How deep is the ocean? How high is the sky? How many times a day do I think of you? How many roses are sprinkled with dew? How far would I travel, to be where you are? How far is the journey from here to  a star? And if I ever lost you, how much would I cry? How deep is the ocean, how high is the sky?

ONE HAND, ONE HEART– from the show "West Side Story" by Leonard Bernstein & Stephen Sondheim. I prefer piano for this one.
LYRICS:
Make of our hands, one hand. Make of our hearts, one heart. Make of our vow, one last vow, even death won’t part us now. Make of our lives, one life. Day after day, one life. Now it begins, now we start, one hand, one heart, only death will part us now.
PRELUDES