No, it equips the same was as it does in Vegas 2, just remember that you have 2 primary weapon slots in Vegas 2 as opposed to the 1 primary weapon slot that you have available in Rainbow Six Vegas. A shield, in either game, has always taken up a primary weapon slot. The difference is that in Vegas 1 it took up your ONLY primary weapon slot, something that they corrected in Vegas 2 by giving you a second primary weapon slot.
As for having to pick up the shield in the story to unlock it in the game; you're right.
"Shield:
This is a special weapon found first in Act 2 "Calypso casino" on the strip or
in the Act 3 "Red Lotus" during the ascent to the Mafia house. With it
equipped, you use only your handgun to fight, but you are protected from the
front from gunfire. Ducking will make you harder to hit, since your legs can
be hit when you are standing. When holding the shield, you cannot rappel,
climb, or hide against walls or obstacles. You also walk a whole lot slower
than non-shield using players."
And there you have it.
"Among the metal ones a messenger will soon arrive. 10001110101 Periodic table with a centerpiece of mind..."
