I assume you want to replace a file with the file v. Your problem now is that the 2nd argument is a folder, and it should be the file you want to replace.
If you have a folder with with 2 files (a.txt and b.txt) and run this:
File.Replace("a.txt", "b.txt", "b_old.txt");
you will end up with b.txt(wich was called a.txt) and b_old.txt(the old b.txt).
If you goal is to just make a backup. I suggest using File.Copy().
System.IO.File.Copy(v.FullName, temppathmoment + @"MinecraftBackUP\" + v.Name + "_backup");
If not you need to put a path to a file, not a folder as the 2nd argument.