No. Just like the frets are farther apart near the nut so will the effect of vibrato. I've found that the 'mwah' effect is brought out more by plucking over the finger board (YMMV).
Minor point: I feel the 'mwah' effect is in the attack of the note, and vibrato is a slight change of pitch , so they really aren't the same thing.
Smaller point: I think vibrato sounds best when it is kept above the pitch. In other words, attack the note and roll up and then back down to the original pitch, don't roll below original pitch. of course... IMHO.
Never confuse beauty with things that put your mind at ease. -Charles E. Ives