Vouchsafe, O Lord

Vouchsafe, O Lord are the initial words of a prayer from the Matins and Vespers service of the Eastern Orthodox, and the former Prime and Compline of the Roman and Eastern Catholic Churches, and for Matins and Vespers of the Anglican, Lutheran, and other liturgical Protestant churches.

Orthodox Church

Matins

For Sunday Orthros this phrasing is employed as part of the Great Doxology:
For weekday Matins, the phrasing is used in the Lesser Doxology.

Compline

Vouchsafe, O Lord is sung as part of the Doxology of Small Compline. The rubrics for Great Compline, used during Lent and Holy Week, and the eves of certain great feasts, specifically direct that the Doxology be read, not sung.

Western Christianity

The Greek liturgical text was imported into the closing of the Western Christian Te Deum, which is used in the Ambrosian Rite at Matins, and in other rites as a special hymn of thanksgiving, e.g. the Dettingen Te Deum composed by Handel to celebrate his patron George II of Great Britain's victory at the Battle of Dettingen.
This usage was incorporated into liturgical Protestant rites at the time of the Reformation. In the 1549 prayer book, the first Book of Common Prayer, which was adapted from the Use of Sarum, it was required for both Mattins and Evensong, but later became optional.

Latin text

Translation from the Book of Common Prayer

Salvum fac pópulum tuum, Dómine, et bénedic hæreditáti tuæ.
Et rege eos, et extólle illos usque in ætérnum.
Per síngulos dies benedícimus te.
Et laudámus nomen tuum in sǽculum, et in sǽculum sǽculi.
Dignáre, Dómine, die isto sine peccáto nos custodíre.
Miserére nostri, Dómine, miserére nostri.
Fiat misericórdia tua, Dómine, super nos, quemádmodum sperávimus in te.
In te, Dómine, sperávi: non confúndar in ætérnum.

O Lord, save thy people : and bless thine heritage.
Govern them : and lift them up for ever.
Day by day : we magnify thee;
And we worship thy Name : ever world without end.
Vouchsafe, O Lord : to keep us this day without sin.
O Lord, have mercy upon us : have mercy upon us.
O Lord, let thy mercy lighten upon us : as our trust is in thee.
O Lord, in thee have I trusted : let me never be confounded.

Protestant churches

Anglicanism

The 1549 Book of Common Prayer collapsed the first three morning offices of the Catholic usage into one service called Mattins, and the latter two into Evensong, the traditional English-language name for Vespers. The other offices were abolished. In this book, the Te Deum, including the Vouchsafe, O Lord, was mandatory at Matins The 1552 Book of Common prayer renamed these Morning Prayer and Evening Prayer, but continued the use of the Te Deum Laudamus, with Vouchsafe, O Lord, at Morning Prayer, with the option of substituting the Benedicite, omnia opera. The 1662 Book of Common Prayer called for either the Te Deum Laudamus or the Benedicite to be recited after the Second Lesson at both Morning and Evening Prayer, thus the Vouchsafe, O Lord could be incorporated into one, or both at the option of the minister. The present Daily Office continues this use, with modernised language.

Lutheranism

Lutheran churches generally incorporate the Te Deum, which includes the Vouchsafe, O Lord into Matins. Vouchsafe, O Lord is not used at Vespers in Lutheranism.