Remember, the definition of jew isn't: "any random greedhead or asshole we hate."
It's a specific accusation about racial (genetic) membership in a specific race.
Here.
My understanding is that VNN's rules are strict about accusing someone of being a jew without evidence.
Evidence, I take it, would be one or more of following:
a) DNA test
b) declaration by the person himself
c) widespread public agreement by jew community (e.g., JTA, ADL).
As Alex Linder pointed out, accusing someone of being a jew is as serious as accusing that person of murder. It isn't a casual insult to throw around because "big nose" "name sounds jewy" "went to X college" "has a lot of money." Those are, as you point out, indications that further research should be done, not proof.