The function f(x) replaces every occurrence of the letter 'e' in the input string with the letter 'V' only if it occurs in the even indexes (0-based).

[CODE]:
```
def fx(input_str):
    output_str = ""
    for i, letter in enumerate(input_str):
        if letter == "e" and i % 2 == 0:
            output_str += "V"
        else:
            output_str += letter
    return output_str
```

Please note that this is an approximation and could have minor differences from the actual implementation.