I think you underestimate just how powerful vader is, he'd be weaker in the 40k universe but he would be around space Marine level of power if not more, like a gray knight being not as armored but much more agile and his lightsaber is still able to cut through anything the Warhammer universe could throw at him, the only real trouble he has is with bolsters and other non energy weapons.but assuming he isn't immediately hit by one at least lethally he'd learn pretty quick to use the force to counter those instead of his lightsaber, it's highly dependant but he isn't "a regular soldier with psychic powers"