Ok here's a logical way for Dave to replace Samy on this Van Halen tour.
After a crappy show, Sammy goes home in his car angry. Seeing that the speed limit is 55 mph, and realizing he cant dive 55, he speeds up to freeway speeds in a residential area. Just then, Gary Cherone, who is on his bicycle, goes out onto the road infront of Sammy. With litle time to react, Sammy swerves and crashes into a telephone pole at over 100 mph. Needless to say, Sammy wont be doing much of anything for a while. Cherone, who is upset that he caused Sammys death, commits suicide.
Now because there are lots of shows left on the tour, Eddie and the guys decide to find a replacement. Oops, two of their lead singers are dead! What to do!
Dave, who has come off of a groundbreaking performace at the Boston Pops is asked to replace Sammy on the tour, since he already knows all those songs.
Even if Dave and Eddie get into ton of fights on the tour, the money that they get from the tour would be enough to make them friends again (not to mention the ladies!).