Update: This site discusses the topic in detail also.
After re-reading, carefully, the section on sacred music in the Constitution on the Sacred Liturgy yesterday and then being given this article by Joseph Swain from Adoremus Bulletin, I have come to the conclusion that, in fact Vatican II contemplated the Mass without hymns-but certainly with music and participation from the people. The article gives some background:
The Second Vatican Council, in the interests of such “active participation”, charged the congregation with singing the actual liturgical texts, but Proper chants are not easy, and so bishops seized upon the more elastic clauses in the Constitution on the Sacred Liturgy and its subsequent instructions and allowed easier and by now much more familiar hymns to substitute.
Then Mr. Swain explains how this became the 4-hymn sandwich we are so familiar with. He goes on to describe the liturgy as celebrated at St. Mark's Cathedral in Venice, where there are no hymns, but Propers and chants are sung by choir, congregation, or both. He lists 5 insights on the experience; here is the first:
First, it clarifies how well chanted music and polyphony accommodate the dramatic continuity that is the Mass. By comparison, hymns and songs, whether Lutheran, Anglican, or some modern type, often seem to halt the liturgical action, except perhaps when they are covering a procession, because their explicit strophic (verse) form and strong sense of meter establish discrete musical structures unlike anything else in the Mass. They stand out, like a skyscraper on a rolling rural hillside. Perhaps that is why, historically, hymns were limited to the Divine Office (Matins, Vespers, etc.), which are essentially contemplative liturgies. The Mass, by contrast, is an action, with its own direction and flow. No music other than plainchant seems to move this action so well because chant, with its free rhythm, most efficiently sets the texts to be sung.
So what about hymns? Well, it appears that they should be sung at other times, other services, other celebrations. So if we want hymns, we need to get together at other times to pray and to celebrate. This sounds good to me!
I certainly remember the May processions we had every year at St. Catherine's parish in Norwood, MA. Most of the Marian hymns we sang that day every year, were only sung then (and for the 4 weeks leading up to the May procession when we learned and practiced them in the classroom).
I think few people sing hymns outside of the Mass-yet that's where they should be sung-as part of our life, our prayer, and our celebrations.
In doing a little research for this post-I know I come lately to these conclusions about the Mass. I remember reading about Mass without hymns at Open Book a few times in the past year-but I didn't pay much attention. (Sorry, I found the link(s) yesterday, but can't seem to find them this morning), but not until I read this article by Joseph Swain did I see it clearly.
Imagine all the music wars that would be ended instantly if parishes implemented Vatican II as the documents proscribe... peace!
Oremus pro invicem!
I am very happy these books are back in print. Especially "Restoration..." which I believe gives a good roadmap for families who want to restore Christian culture in their homes and then in the larger local community. 
