I think anyone trying to "fake" having magnet pull is fooling themselves. It's beyond obvious if your steel type is trapped (scizor won't u-turn, he'll just switch).
Is assault vest really better than leftovers? Which special attackers are you tanking hits from? The only ones I see are greninja without HP fire. Most other top special attackers eat zone for lunch, right? (serious question, not trying to be snarky)
There are actually quite a few strong special attacks Zone can take vested that it can't otherwise.
252 SpA Life Orb Alakazam Focus Blast vs. 252 HP / 0 SpD Assault Vest Magnezone: 255-302 (74.1 - 87.7%) -- guaranteed 2HKO
252 SpA Life Orb Protean Greninja Hidden Power Fire vs. 252 HP / 0 SpD Assault Vest Magnezone: 159-190 (46.2 - 55.2%) -- 68.4% chance to 2HKO
252 SpA Life Orb Gengar Focus Blast vs. 252 HP / 0 SpD Assault Vest Magnezone: 250-294 (72.6 - 85.4%) -- guaranteed 2HKO
252 SpA Life Orb Thundurus-T Focus Blast vs. 252 HP / 0 SpD Assault Vest Magnezone: 270-320 (78.4 - 93%) -- guaranteed 2HKO
+6 252 SpA Thundurus Thunderbolt vs. 252 HP / 0 SpD Assault Vest Magnezone: 207-245 (60.1 - 71.2%) -- guaranteed 2HKO
252 SpA Life Orb Thundurus Focus Blast vs. 252 HP / 0 SpD Assault Vest Magnezone: 242-286 (70.3 - 83.1%) -- guaranteed 2HKO
252+ SpA Heatran Flamethrower vs. 252 HP / 0 SpD Assault Vest Magnezone: 236-278 (68.6 - 80.8%) -- guaranteed 2HKO
Not that you'll switch into any of these, but if the opponent tries to revenge Zone with one of these, you tank their attack. Sash Zam wins, however, if Fail Blast hits twice (read: almost never).
And you're right about Magnet Pull. I should have been more clear. I meant that most Zones are HP Fire / Ground Magnet Pull with Specs to trap Steels, and it's normally a fairly obvious switch on both sides. So Zone can scare non - Shed Shell Skarms off the field without even switching in. It's just his presence on your team that makes your opponent be careful about switching their Steels in.