I really want to do a two-part post on vegan suits (part one: women; part two: men), but I am having trouble finding vegan full-suits for men. I will keep searching -- any help would be greatly appreciated.
In the meantime, I stumbled upon some vegan office-appropriate pants for men from Bonobos ("pants for real guys" as they call themselves). For a business casual office, they would be perfect paired with a collared shirt and a sport jacket or a collared shirt and a nice sweater.
I especially enjoy these "clean slates" (pictured left), although they may be a bit too casual. 100% brushed cotton twill; $120; they only come in the pictured slate grey color.
These "congos" (pictured right) are a bit more dressy. Also made from 100% brushed cotton twill; also $120; they also only come in the pictured safari green (which looks like a deep tan to me despite the color label).
These "graham slackers" (deep khaki color; $110) and these "the brothers grinn" (dark brown color; $120) also likely are business casual office-appropriate.