The function reverses the given input string and replaces all vowels with their previous letter in the alphabet.

[CODE]:
```python
def f(x):
    vowels = ['a', 'e', 'i', 'o', 'u']
    output = x[::-1]
    for i in range(len(output)):
        if output[i] in vowels:
            output = output[:i] + chr(ord(output[i])-1) + output[i+1:]
    return output
```