Is it true that vaguazhang is a martial art that was designed to defeat other martial artist and it is an art which defeats all styles?

Yes, No.

(My elaborate answer died when my monitor went to "the blue screen of death" -- so here's the "Reader's Digest" version):

Ba Gua Zhang is a martial art designed to defeat other people -- whether they be other martial artists or merely unskilled ruffians.

No style on earth defeats all other styles, nor defeats any other style for that matter (so don't bother wasting your time & energy looking for one that does). For instance: Judo & Boxing are different styles; a Judoka doesn't always defeat a Boxer, & a Boxer doesn't always defeat a Judoka.

Effective martial art is all about ability to apply appropriate body mechanics -- regardless of "style."

Good answer Mike (further elaboration would only have spoiled the moment).

amen.

This spiritual moment is brought to you by the proud folks at Tim's Shen Wu Academy. If you too would like to experience a truly Zen flavor in your martial training. Zen I suggest you head on down to Tim's. You too can find what you thought you had all the time. Don't delay, do it today! Once you try Shen Wu, you just don't have the heart for anything less!