You can't possibly know how much the flights are this far out. So much can and will change, its almost 2 years away. Airline pricing is a dark art, the price you see today is not necessarily reflective of what you should be expecting to pay.
Trust me, I spend way, way too long monitoring transatlantic flight prices - with so long to plan and wait, you WILL get to LAX cheaper than you will get to Vegas. You can't just do a random spot check and decide from there. LAX is a more competitive market served by more direct flights with more carriers.
It will be cheaper to fly to LAX and it will make literally no difference to your trip - as you are going to LA anyway. Vegas is premium priced because it attracts the stag do crowd, LAX is usually so much cheaper that even if you are only going to Vegas it's usually cheaper to fly to LAX and drive a Mustang across the desert to Vegas
FWIW there is actually nothing wrong with Norwegian, they fly the 787 to LAX which is somewhat better than the 20 year old 747's Virgin fly to Vegas