Years ago, a small(er)-time promoter gave RVD his name because he said that Rob looked remarkably like Jean-Claude.
The name stuck.
RVD has also made some low budget/quality MA flicks - something along the lines of Cynthia Rothrock's.
I believe I also once read that he acted as a double for JCVD in one of his films.
While on the topic, Rick Martel was once offered to act as a double for Sylvester Stallone, but turned it down.