Is there a substitute Greek letter for V?

There isn't a direct Greek letter substitute for the Latin letter "V". The Greek alphabet doesn't have a letter with the same sound or shape. The closest approximations would depend on the *sound* you intend "V" to represent.

* For the /v/ sound: Ancient Greek didn't have a /v/ sound. Modern Greek uses either the letter β (beta) or sometimes μπ (mi-pi) to represent it, depending on the context and dialect.

* For the Roman numeral V (5): The Greek equivalent would be Ε (epsilon), representing the number 5.

So, the answer depends entirely on the context. There's no single "substitute".