Are you looking for Latin or English?

If you are looking for Latin: The only OF chant book for the Roman office to be published thus far is ARII. The only thing you can do is try to find the updated Ordo Cantus Officii and go asearching through the older Antiphonals and Libers until you find what you need. NB, the Breviary is mostly useless for this, because the texts don't match between it and the OCO: there are some neo-Gregorian Latin antiphons that someone pointed out here (I think from the Cathedral in Prague?), that are settings of the Breviary texts, but I can't seem to find it at the moment.

If you are looking for English: The best thing I can tell you is to get a copy of Fr. Weber's "Hymnal of the Hours", and then to scour his website for anything you might find.

This is a monumental task, and I'm not sure your pastor knows exactly what he's getting you into -- you will end up collating your own AR III, AR IV, AR V, etc.

We have been singing Vespers (OF, English) for a few months now, and more time has gone into that aspect of my job than most anything else. The amount of time needed to put together books for a 30 min. Vespers (and it's only 30 min.s because we pad it) is disproportionate.

Best of luck to you.

We are starting up OF vespers on an occasional basis. I have been setting the english text from LoTH to the chant melodies found in ARII. This has been an arduous process. So far, I have it done for Palm Sunday Vespers II and Pentecost Vespers II. I am currently working on the Office of the Dead.

If you are looking for "easy" english - the Mundelein Psalter is there and mostly done already, but the tones are extremely boring. You could always use one of their boring tones for the antiphons and then set the psalm to real gregorian psalm tones and point the text. I sing occasionally with a friend that uses a combination of Mundelein Psalter tones and Conception Abbey harmonized SATB.

Unfortunately as Salieri said, the resources are just not there to actually "chant" vespers in english. Fr. Weber has good stuff, though lots of it is using Mundelein or Conception Abbey tones. Richard Rice has done some excellent stuff, and might be inclined to chime in.

Get ready to dig in and work!

I really do need to purchase LHG, but it's so expensive. Perhaps one day...

The Mundelein Psalter is a good place to begin such a project. The tones will become boring if sung daily, but if only used occasionally they will prove to be accessible to a congregation.

If you desire something less repetitive but still reasonably accessible, I would suggest contacting Saint Meinrad and asking what resources may be available, they have much that is not online. While the monastic use in the abbey church is very beautiful, if you are aiming for congregational participation, the way found in the seminary chapel there would perhaps be a better choice (they use the same psalm tones, but beyond that the two ways diverge a bit). I have participated in a parish Vespers done in close accord to the methods in use in the seminary chapel, and it seemed to work rather well.

While the preservation of Gregorian melodies as part of the life of the Church is valuable where possible, I am not certain how well those melodies will work with the English texts.

On the specific question in the last post of arranging an extended Vigil from the Office of Readings:
Having just re-read the relevant parts of the General Instruction of the Liturgy of the Hours: no. The first portion of the office of readings, including the hymn, psalms, and readings with proper responsory are done, then the canticles with antiphon are added, then the Gospel, then the homily (optional), then back to the usual office of readings format with the Te Deum, prayer, and conclusion.

A respectful correction to @Salieri, but the Antiphonale Romanum II is not the only OF chant book for the office. Rather than being a second edition to the Antiphonale Romanum of last century, it is in fact the second volume following Antiphonal Romanum I, or as it is actually known, the Liber Hymnarius, which @smvanroode has referred to.

The LH has - as would be expected - every hymn you could possibly need. It also has every Invitatory antiphon and corresponding settings of the invitatory psalm. It has some responsories too, but I can't remember what they were for (I haven't got my copy on me) - perhaps Office of Readings?

And I would love to hear of any projects to set The Divine Office (the UK/Australia/[non-American-actual-English-:p] translation) to the correct or closest-to-correct chant melodies... These other projects using LoTH are all but useless to us unfortunately.