That's not valid MATLAB syntax. It looks like you may be trying to create variables with sequentially numbered names (v1, v2, v3, ...) and if you are that's strongly discouraged. See this Answers post for some of the reasons why that's discouraged.
If you describe what exactly you're trying to do with that for statement we may be able to offer some suggestions. It may be as simple as calling cumsum to create a vector v, but I'm not certain.