Note also that Vader isn't generally on the front lines, with the exception of the first Battle of the Death Star.  In coming aboard the *Tantive IV*, he boarded while his troops were mopping up after the vessel was mostly secure.  He did the same on Hoth, and while he entered Cloud City before it was secure, he was entering a civilian environment with an overwhelming military force.  This is not out-of-line with the behavior of a General in any army.  Yes, there may be still be fighting occurring when a General enters into a newly-secured area, but he usually enters into it **after** it is mostly secured; unless things go badly awry, he's not on the front line.