a char(only one) stands with vengeful ,and before I have the chance to draw card,the first player kneel it ,can I draw?

Absolutely. The character still stood for Vengeful even if he is knelt some other way by the time you trigger the Response to draw a card.

with this,do you think link on wendamyr can trigger after he kneel to save himself and stand?

Definitely. He knelt, after all.

Highlighting the difference between "after a character is knelt" and "if a character is kneeling."

This is kind of the similar line of thought when people ask if standing a character in a challenge removes them from the challenge. In this, just because a character is now standing does not mean they were removed from the challenge unless the effect says it does. The same as if you kneel a character and they were then stood, knelt, killed, discarded, set of fire, blanked, etc… during the response window the fact remains that you did indeed kneel that character and satisfy the requirements of the effect.

Though I am curious what cards in particular you are referencing to have the outcome you were describing.