Vivica A. Fox -- The Biggest Will Smith Lie You Will Hear All Day ...

Vivica A. Fox should've gotten busted this weekend ... for excessive chugging of Will Smith flavored Kool-Aid -- after actually claiming "After Earth" is a great movie.

Vivica looked our camera guy dead in the eye and said ... unsolicited mind you ... "I totally recommend for you guys to go check it out this weekend starring Will Smith, Jaden Smith. It's awesome."

Vivica couldn't stop gushing ... you have to hear the rest of the over-the-top promotion for her old "Independence Day" co-star.

It's kind of hysterical considering 'Earth' has been universally panned by critics who've said things like, "Summer 2013 has its first bomb" ... and, "Yes, it's that bad."

So, who's lying ... Vivica or the critics?