Vigenère cipher with sha512 and a little foresight turns to one-time pad. How safe can be?
Is this a real one-time pad?


No, it's not a one time pad; as the keystream is not generated from a truly random process, hence you don't get the informational security guarrantees that you'd get from a true OTP.

Instead, it's a stream cipher; in particular, counter mode (with SHA512 is the "block cipher"; in counter mode, the block cipher needn't be invertable).

It's actually decent at providing privacy (albeit not very fast at it); however, it still would need some integrity protection (unless you're absolutely positive that an attacker cannot alter the ciphertext)