unless you spend a lot of money, zooms aren't usually as good as even the old primes are... i'd replace the 24mm, and get a 28mm as well, like dms says.
those old vivitars are fairly cheap here in the states, can you find 'em in singapore?
i just tested a 24mm vivitar(actually tokina, 37xxxxxx serial number) prime on my a7r, and it was surprisingly good at ~f10, much cleaner on the sides than my smc pentax 24/2.8... unfortunately the aperture leaves are gummed up, gotta fix that :-/
here is the link to the vivitar manufacturer database, by serial number:
https://www.cameraquest.com/VivLensManuf.htm
for 28mm lenses, the smc pentax-m 28/3.5 is the best i've tested so far, at ~f10, for landscapes.