Using varargs like Andre tried doesn't work because pushing a va_list isn't the same as pushing a list of arguments. So I think we need assembly. The assembly I gave earlier is wrong because it cleans up the stack, which it's not allowed to do. I've attached an updated patch to http://bugs.winehq.org/show_bug.cgi?id=26688 with a _vcomp_fork that works for simple loops with and without bound variables.
I guess the next step is porting the serial stub for _vcomp_fork to 64 bits.