Steven,

OF or EF? (Ascension's Vigil isn't the same in the two situations.)

I would sing say the (I) Vespers Hymn / Matins hymn etc. Otherwise sing Hymns of the Feast of the day St. Bernardine of Siena (I have not checked if there are Proper Hymns)

Rogations Hymns is an interesting option and I am sure there are suitable examples in the Analecta Hymnica, but I don't know if anyone has produced them in singable editions.

Are these additional to the propers or substitutes? Vocem iucunditatis, Benedicite gentes & Cantate Domino show a fairly consistent note of sung praise rather than any other theme.
If Mass is celebrated for the Rogation Day, and then later for the Vigil it would not seem appropriate to use similar hymns. But if this is the only Mass, and later in the day, it might feel right to start with a Rogation hymn before the Introit.

In another thread: The Lamb's high banquet we await / Ad coenam Agni providi [tune: St. Croix] - (Charles H. Giffen)

Also: "Jesus, Lord of Life Eternal" ("Canon for Ascension" Ode IV, by Joseph of the Stadium, transl. J.M. Neale) set to my tune Beaudry.

The Lamb’s High Banquet We Await is set to PUER NOBIS in the Lumen Christi Hymnal.

From an EF perspective, some opinions which probably won't help much:

* I was unable to find any music specifically written for the Vigil of the Ascension in any sources -- no medieval sequences or hymns or poems. (This seems a lacuna, for there is no shortage of material for days within the old Ascension octave.) As you note, the Proper for the Vigil Mass is simply for the 5th Sunday after Easter with different readings. This seems to hold true in the Office of the Vigil as well. The only thing proper for the Vigil is the readings, both in the Office of Matins as well as in the Mass.

* About the readings: The Epistle and Gospel for the Vigil Mass are absolutely sublime. I cannot think of any English hymns which really fit either of them well, but if you can, I'd recommend considering them. Otherwise, you can program for Easter V, e.g. seasonal Easter music. And you might consider organ music instead.

* Since the EF is infrequently celebrated and even less so with music, and now it seems commonly for congregations who love all sorts of pious customs and traditions, "neighboring" and "suppressed" days and seasons are often used to inspire the music at a particular Mass/office. So, for example, if the Vigil Mass is celebrated after 4pm and is the only EF Mass this week, one might consider Ascension music because it's your only opportunity. Likewise, if there is no second Mass of Rogation on the Wednesday, or no Mass said votively for the saint, one could incorporate some elements of the penitential season, especially in Latin in the middle. Rogation and the Vigil grew up separately: Rogation is in purple vestments, with a procession at which hymns of joy are explicitly forbidden, but the Vigil is in white, indeed the last Mass of the Easter season (with Easter preface).

* Guéranger, who does not remark on the Vigil in his Liturgical Year, reminds us that divine Providence has decreed that the Ascension should always fall in the Marian month of May, about whom numquam satis, etc.

There must be composers up to the task of using SIP orders to compose appropriate music.

Or even hymn-poets...