If i need to cook vegetable at high temperature of 183F, does it require the bag to be fully vacuum sealed?
You can use the water displacement method to get the air out of the bag. You want the air out of the bag or the cooking will be less efficient / uneven due to that air pocket.
About the only time you want air in with your food is when you’re cooking in jars with your sous vide (creme brûlée, dulce de leche) - even then, you want the jars only “finger tip tight” so the air can escape.
Yup @fischersd is totally right! If you want a visual, check out this video for how to do the immersion method: Video: The Immersion Method
Getting a good seal on veggies is tough because there are just so many air pockets. The floating bags blog post on blog.anovaculinary.com was helpful. Now I just add a heavy knife to the bag and throw a heavy bowl on top. Since veggies are so quick, getting hacky with it is pretty easy and functional.