"Vegetables are not tasty."
I don't like that sentence either. It is promoting a bad habit! Let's go healthy! More veggies and less meat!
맛없 it seems like the ㅅ should sound like s since it is followed by vowel 어
It should. But it doesn't. 맛없다 is an exception, the correct pronounciation of this word is like 맏없다 or 마떲다. In basically every other case, what you wrote is true, that is ㅅ is pronounced as 's' when followed by a vowel.