SiaoAhBeng, u got your ans here.
Just like posted, it will look great with white shirt. Actually white shirt is the best and must have item all men must have.
It will look great in working pants and even jeans, if its nicely pressed and cutting is just nice for u.
And yes abt the size and colors of the vest. I seen my director wore a red vest over white long sleeve shirt and jeans. FYI, he is in his late 50s but he looks amazing in that!
Try to be more daring with the colors of the vest, coz your shirt itself is already quite bland...
But try not to have strong colors for both shirt and vest...
As for long pants, most guys usually start off with brands like G2000. Unless u are smaller than sz28, then u will need to go tailor. Like me...