"Weng" syllables rhyme with the fnal -ong According to Wiedenhof's A Grammar of Mandarin, page 44,


  "Weng" syllables rhyme with the fnal -ong [ʊŋ]


However, he'd specified weng as [wʌŋ], to add


  Weng displays the same type of variation as the fnal -un: it may lose
  its rounding toward the end, [wəŋ].


Page 66 reads


  there's free variation between [wʌŋ] and [ʊŋ] for both fnals, with
  complementary distribution


Can somebody please clarify the apparent ambiguities? 

In the second paragraph, what does end refer to?
In the second paragraph, what does end refer to? 


When I speak, I tends to do something like this /wʊ-ɤŋ/. I have never noticed this behavior before. For the word 老翁 (old man), if you say it either way: /lɑʊ.wʊŋ/, /lɑʊ.wəŋ/, /lɑʊ.wɤŋ/, /lɑʊ.wʊəŋ/, etc, they are all interpreted as the same word, thus "free variations". This depends on the individual accent.

I have never studied rhymes seriously before, so if your book says "weng" rhymes with "tong", then they must be using the /wʊŋ/ way of speaking. For example: 通風 (air circulation) may be pronounced as /tʊŋ.fʊŋ/ or /tʊŋ.fɤŋ/, but never as /tɤŋ.fɤŋ/. I personally say /tʊŋ.fɤŋ/. 

As for your last quote, I don't understand how the free variation has complementary distribution.