Answer 1

You almost had it.
assign it again to s and print the result after loop

s=input()
vowels="aeiouAEIOU"
for ch in vowels:
s=s.replace(ch,'*')
print(s)

Answer 2

vowels="aeiouAEIOU"
for ch in vowels:
vowels=vowels.replace(ch,'*')
print(vowels)