Eh, regarding the scene in R1, Vader clearly brings troops with him. Its evidenced by both his line about preparing a boarding party and the fact that Stormtroopers show up next to him as they watch the Tantive IV fly away. As to why he takes on the rebels single handedly? Perhaps he had his team split up to search the ship faster and he just happened to be the one who found them first. He didn’t really have time to wait for backup. It’s an entirely different scenario than the opening of Star Wars where he’s cornered and captured the ship and can take his time letting the troops storm it.
This