What does "number f**king ten" mean?

I was watching "Casualties of War" today.

One of the characters, a soldier, interrogates a suspected VC (Viet-Cong) old man by asking him "Are you VC? Number fucking ten?"

Is "number fucking 10" some kind of military slang, or maybe is it some mnemonic way to ask something in Vietnamese?

A quick online research shows that "number 10" is indeed part of 60s' Vietnam War jargon, meaning "the worst" and being the opposite of "Number 1": "the best"; according to glossaries in Ray's Web Server and Institute for Advanced Technology in the Humanities of Virginia.

I think, in the context given in the question, it means American in a pejorative sense (see @armen-Ծիրունյան's answer), with partial rhyming slang (ten/-can).

The interrogator already thinks the guy is VC, and is using language to try to get him to admit that. If the guy balks at the pejorative term, then that's more of an indication that he sides with the Americans than just simply agreeing to the pejorative term.

This analysis of the "poem" suggests it refers to Americans who raped / made Asian women pregnant:

This glossary also suggests American: