The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) discusses the profound significance of the verse in Tehillim (90:17), "Vihi Noam Hashem Elokenu Alenu U’ma’aseh Yadenu Konena Alenu U’ma’aseh Yadenu Konenehu." He writes (listen to audio recording for precise citation) that there are "lofty and exalted" meanings to this verse, but one should at very least ensure to have the straightforward meaning in mind when reciting this verse before praying or performing Misvot.
Literally, this verse means, "May the pleasantness of Hashem be upon us, and our handiwork shall be established for us; and our handiwork – He shall establish." Based on the Zohar, the Ben Ish Hai explains this verse as a prayer that God should consider our prayers and Misvot complete, even though we cannot have all the lofty intentions of the deep meanings underlying the prayers and Misvot. We are unable to have the intentions needed for our prayers and Misvot to proceed to their desired destinations in the heavens. We therefore ask Hashem that He should "establish" our prayers and Misvot, meaning, that He should "complete" whatever they are lacking due to our inability to have all the deep intentions. They will thus have the desired effect in the upper worlds, with the power that they receive from the Almighty.
The Rashash (Rav Shalom Sharabi, Yemen-Israel, 1720-1777) offered a different explanation of this verse. The sins that we commit cause us to lose the light of sanctity that shines upon us as a result of the Misvot we perform. The spiritual effects of our Misvot are sabotaged by our sins. We therefore beseech God to "establish" our good deeds, to restore their full spiritual power despite our wrongdoing, so we can again benefit from the Kedusha generated by our Misvot.
The Ben Ish Hai writes that it is proper to have both these meanings of the verse in mind each time we recite this verse, and that this verse should be recited before every prayer and before every Misva we perform, such as before donning Tallit and Tefillin, before giving charity, and so on. There are those who recite this verse twice, and having one of these intentions during each recitation. The Ben Ish Hai writes that this is a proper custom to follow.
It should be noted that although the custom in Halab (Aleppo) was not to recite the "Le’shem Yihud" paragraph before praying or performing Misvot, the Halab community did make a point of reciting "Vi’yhi Noam" before every prayer and Misva. We should thus certainly perpetuate this practice and recite this verse before our prayers and Misvot, having in mind the two intentions discussed above.
Summary: It is proper to recite before every prayer and before performing any Misva the verse of "Vi’yhi Noam." One should have in mind through the recitation that God should consider our prayers and Misvot complete even though we cannot have all the deep intentions associated with them, and that they should retain their spiritual power despite the sins that we commit. Some have the custom of reciting the verse twice, each time with a different intention.
