Vigenère cipher with sha512 and a little foresight turn into a safe one-time pad?

As an encryption one time pad: It has a very simple solution space where the pseudo random sequence starts. Say you know the file type encrypted (pdf, doc), then you can get away with probably bruteforcing 1-3 letter at start to obtain the 'increasing number'.

In the author's example, the increasing number starts at zero, so the bruteforce is 28 guesses to check the first letter (assuming you know the encrypted block and the filetype)

As a hashing sequence generator, all the solution will be chained from (0..28)+i, so... all it takes is to hash to find 28+i and compare with the generated sequence. Then you can predict the sequence from that point on, which is deterministic.

Disclaimer: I'm not crypto expert, it's 2 am and am zoning out, etc...