"like from LA to Vegas?"
It's complicated. For your specific example, the answer is technically no, you cannot. When crossing state lines, whether at ground level, or at 35000 feet, you cross a border zone that is federally managed and protected. This means that if you were to fly with cannabis from one 'legal' state to another, even if each share a common border, then you are in breach of federal law.
If, however, you are flying within a legal state, then you are fine so long as you observe limits on possesion quantity and do not stow lithium batteries in the luggage hold (i.e; vape pens have lithium batteries).
No, As per my expereince no need to fly with weed. It will create some problem in some state due to state legalities.
Good news is, when you search about online weed in each state; there are many options to buy online on that particular state. And there are many manufacturers who provide their products on all 50 states.