The immediate answer, given the date of July 1941, is it refers to the rallying symbol of the allied forces, V for Victory. (Or Victoire in French, or Vrijheid for freedom in Dutch). It was originally thought up by a Belgian, actually, hence the consideration of both languages, given it’s a split country. By the middle of 1941, it was popping up most everywhere, anywhere from Winston Churchill’s two fingers to graffiti in occupied nations. Even in music, that familiar “Dah Dah Dah DUM” of Beethoven’s Fifth (which is also in the opening sequence to the WW2 D-Day movie “The Longest Day”) is a V, as the Morse code for the letter is dot dot dot dash.
With broadcasts coming over from England referring to the V, resistance groups popping up V propaganda markings, and even the Prime Minister dishing out Vs with his two fingers (sometimes in the FU format), one can see some amusement value in the idea of British propaganda showing up even in the Fuhrer’s meal, portending his doom...
In a July 1941 cartoon captioned "crisis in Berchtesgaden," Hitler is aghast to discover the letter V in his alphabet soup. The British had made the V sign an internationally recognized symbol of victory (and of anti-German activities).
The cartoon refers obliquely to a series of cartoons in the New Yorker dealing with highly unexpected situations... In August, Hitler appears as a mighty hunter carrying his outsize moose ("vanquished Europe") home on his shoulders only to have the moose bite him in the rear - "Sabotage". Like the Hitler of alphabet soup, this Hitler is decidedly unjaunty.
- Excerpts from Dr. Seuss Goes to War, Minear, Richard H., p. 73
Your source is from the UC San Diego Library (The Geisel Library), which was renamed in 1995 in honor of Seuss, Theodore Seuss Geisel, and his wife Audrey (who happened to pass away in December at the age of 97). The collection that your source is from - "Dr. Seuss Went to War" - is named as an homage to Minear's book, published in 1999.
As for the content of the cartoon, we are perhaps more familiar with the usage of "V" nearer the end of the war as in V-E Day and V-J Day, or Victory in Europe Day and Victory in Japan Day. However, victory for the Allies was still seen as improbable to the average person in July 1941, when the cartoon was published in PM.
Additionally, the title of the cartoon comes from Berchtesgaden, the town situated near the famous 'Eagle's Nest' fortress in the Bavarian Alps, a meeting place for the Nazi elite.