The downfall and dispersion of our people is painstakingly detailed in the "tochecha" of parshas Bechukosai as the consequence of ignoring Hashem's many messages to us. Nevertheless the parsha leaves us with the uplifting and soothing pasuk (26:42), "I will remember My covenant with Yaakov and also My covenant with Yitzchak and also My covenant with Avraham will I remember, and I will remember the Land."
The medrash points out that upon careful study there is indeed more good news hidden in the nuances of the text. Oddly enough "Yaakov", which is usually written without a vov, appears here with a vov between the kuf and the beis. Rashi points out that the extra vov variant spelling appears no more than five times throughout Tanach. The rabbis correlate this with a variant spelling of "Eliyahu" which is missing its vov five times throughout Tanach.
Chazal interpret this to indicate that Yaakov took a letter from the name of Eliyahu as a security, to be returned to Eliyahu, when Eliyahu announces our geula, our return to Israel and the Beis Hamikdosh. Presumably remembering his own fear of becoming entrenched in Mitzrayim and forever concerned about the welfare of his children, Yaakov asks for this security so that he can be assured of his children's return.
Why five vovs? I would suggest that the five vovs become the metaphor in our parsha for our inflexibility and inability to introspect and reconsider. After all, the tochecha organizes our punishments around five stages, each one marking our callous lack of response to the messages of the previous punishments. Each stage is introduced with a vov: "And if you will not listen to me…"(26:14), "And if during these you will not heed me…"(26:18), "And if you walk with me in a casual manner…"(26:21), "And if despite these you will not be reprimanded…"(26:23), "And if with all this you will not listen to me…"(26:27)
What is the significance of securing Eliyahu's promise? The Potoker Rov explains (Beis Aharon, page 162) that indeed should we merit the geulah we would not need any assurance of Eliyahu or the security that Yaakov exacted form him. However if chas veshalom we would not merit redemption and we can be found guilty of the vovs of the tochecha, we can still be assured that at a preordained time we will nevertheless be redeemed and returned to our earlier glory (see Sanhedrin 98a).
I believe there is still more communicated through the vov. Grammatically, the vov can be in place of "and", stringing together lists and sequencing events. In the case of the tochecha, the vov shows stubborn stagnancy and obstinate hubris which in and of itself indicates spiritual decline. On the other hand, the same vov can grammatically transform a word from future tense to past tense. This use of vov serves as a metaphor for the realization of dreams and aspirations, and for enabling us to build on those dreams as if they were already in hand.
Yaakov assures us that even if we are entrenched in the obstinate vov of Bechukosai, and do not employ the later vov to transform ourselves and aspire to spiritual greatness, we will still be returned to our earlier glory.