From looking into it, while vb6 allowed the program to read/write to a specific block within the Rand file, .Net that idea is a little different in that you'd want to read the entire file into memory and parse it out into a collection of a class that you make that matches the structure of the Rand file's records. Then when done the program takes that collection and writes it all back out to the file.
If your program is the only one using this file, maybe instead of dealing with keeping the data in it you could read the file in, then write the data out as xml which is far easier to work with in .Net and will be more compatible going forward.