Yeah, Let The Killing Begin is an excellent intro.....although very little of it's effectiveness is attributable to Vital Remains.
I don't know, there ARE so many of them that unremarkable ones can seem redundant, or just go unnoticed entirely....but I think its important therefore to consider such attempts at atmosphere or thematic validity or contrasting moods or whatever the intros happen to be there for within and only within the context of that album. Very difficult to do I know with metal music, but sometimes such tracks really will complement the music in some way....
Most often though, they seem to be just simple pieces who's presence is merely there to give contrast and make the opening music hit harder, or to establish an inquisitive mood of mystery (suitable to metal's individualistic emphasis) that then begins exploration with the music starting. In other words, a passage intended to 'open your mind'.