I grew up in Hamburg and explain it like this:
Some reasoning was announced:
Wegen Regens gehe ich nicht raus!
Because of rain i will not go out!
But obvious reality dismisses the reasoning from there on:
Von wegen Regen! Ich geh jetzt raus!
No rain after all? I am going out!
I appreciate the responses to my post, but they didn't address the root of my question. I did think the comparison of "von wegen" with "as if" in a brief comment by @HenningKockerbeck was quite insightful, but it still lacked the connection to the words "von" or "wegen" that I was looking for.
So I did some more searching and came across an explanation in ...
Note that von by itself can have a causal component.
Seid ihr auch manchmal einfach nur noch müde von dem ganzen Elternstress? Dann gönnt euch doch erst einmal eine ausgiebige Runde Schlaf. (1)
Mr LeFroy trat hinter mir in das Halbdunkel der Hütte. Diese bestand auf den ersten Blick nur aus einem winzigen Raum. Von früher wußte ich jedoch, daß es ...
I would translate it as "Venice, my ass!" or, less salopp, "Venice, they say...", or, as in your suggestion, "So much for Venice..."
"No way this is Venice!" is another alternative, not quite dissimilar in the meaning from the first interpretations.
Von wegen as such is an idiomatic expression whose meaning cannot be directly extracted from the meanings of ...
This text is part of the refrain of a song written by the Viennese comedian Georg Kreisler in 1958. Since it is written in Viennese colloquial speech it is not standard German.
The song is on Youtube, and from 0:40 to 0:56 Kreisler sings:
Schau, die Sonne is warm und die Lüfte sin lau.
Gemma Taubn vagiftn im Park.
Die Bäume sind grün und da Himml is ...
Gehn is a contraction of gehen ("to go"), while ma is a slurred pronunciation of mir, which Wiktionary defines as:
(regional, dialectal or colloquial) Alternative form of wir ("we")
So the (quite sordid) question is rendered in a colloquial style:
Wanna go poison pigeons in the park?
In standard German, it would be:
Gehen wir Tauben im Park ...