Baal HaTurim explains that the two Parshas are read together on Shabbos in some years and on separately in other years. The above mnemonic tells us a formula:
When the first day of Rosh HaShanah (המלך the day of the King) occurs on the second or third day of the week (ב"ג), then the Parsha of Netzavim is separate (broken (פת from VaYelech. When Rosh haShanah occurs on the fifth day or Shabbos (the set calendar does not allow for Rosh haShanah to occur on the first, fourth or sixth days of the week לא אד"ו), then Netzavim and VaYelech are read together.
Since Netzvaim is always read on the Shabbos prior to Rosh haShanah, some years will occur when VaYelech is not read that year (read on the Shabbos prior to Rosh HaShanah together with Netzavim, and then the following year might be read separate from Netzavim after the following Rosh haShanah).
The Last Day of Moshe
דברים לא ,ב: וַיֹּאמֶר אֲלֵּהֶם בֶן־מֵּאָה וְעֶשְר ים שָנָה אָנֹּכ י הַיוֹם לֹּא־אוּכַל עוֹד לָצֵּאת וְלָבוֹא ה' אָמַר אֵּלַי לֹּא תַעֲבֹּר אֶת־הַיַרְ דֵּן הַזֶה:
Baal HaTurim remarks the word ולבוא is written full (with a vav=6) to indicate that Moshe meant that, “I am no longer able to come into (the depths of understanding) the Six Orders of the Mishnah.”
The Gemara Sotah (13a) interprets the phrase לצאת ולבוא as a reference to familiarity with the gateways of wisdom. The Baal HaTurim explains that the “gateways of wisdom” are the Six Orders of the Mishnah.
The Six Exiles
דברים לא ,יז : וְחָרָה אַפ י בוֹ בַיוֹם־הַהוּא וַעֲזַבְת ים וְה סְתַרְ ת י פָנַי מֵּהֶם וְהָיָה לֶאֱכֹּל וּמְצָאֻהוּ רָעוֹת רַבוֹת וְצָרוֹת וְאָמַר בַיוֹם הַהוּא הֲלֹּא עַל כ י־אֵּין אֱלֹקי בְק רְ ב י מְצָאוּנ י הָרָעוֹת הָאֵּלֶה
Baal HaTurim observes that the gematria of the word ומצאהו (and many evil things and troubles will befall them) is missing a 'Vav' (after the 'Alef'). This hints at the six stages of exile that Yisrael experienced: three at the hand of Sancheriv (exiled the tribes of Reuven, Gad, and Manasseh; then exiled Zevulun and Naftali; and then the remaining tribes of the Northern Kingdom). In addition the three exiles executed at the hand of Nevuchadnetzar (who exiled Yehudah and Binyamin three times: with Yehoakim; with Yehoiachin; and with Tzedkiyaha).
