Sunday evening has more people out than the other nights, and it can get pretty crowded. I think it has gotten to the point where almost every night is active, but Sunday is still the most active.
Getting there about 6 pm should work for getting a table if you're planning on eating dinner on the Malecon and watching the sunset at 7 pm. Not all the vendors and street performers may be set up but they will be by the time you're finished with dinner.
As far as dinner from the vendors...you'll see hot dog and Elote vendors, and once you've tried them you probably won't want them again.
Elote is very good when made with the right kind of corn, but the corn they use is field corn and is difficult to digest, and chew.
You'll also see cups of cut up hot dogs which are very unusual tasting and the Mexicans like them a lot, but I don't know a lot of tourists who crave those particular hot dogs lol. So, in light of that I don't suggest dinner from the vendors, but it's always interesting to try these things.
There are fruit and dessert vendors with pies and cakes that are very good, and there are two crepe carts...both excellent and different from each other. :-)