I am taking my dad to Vietnam for vacation and due to my poor scheduling I am arriving from Vietnam landing in LAX and then heading straight to Mexico for a wedding. Will I need to go through customs and back into the international terminal, or can i stay in the international terminal and just proceed on to Mexico? I am trying to figure out how long of a layover I need. My dad is continuing on to PDX to go home, any idea how long it will take for him to get through customs? Neither of us have global entry and I do not think I have time.to get it. We leave in February. I really appreciate any help someone can give, I have never done anything like this and am kind of lost.

USA does not have the concept of "airside transit".

You must formally enter USA with all your checked & carry on baggage. Passport, immigration, customs etc
Even passengers on the same flight such as Air NZ's AKL-LAX-LHR flight have to do this.
A very few USA airports have air side luggage transfer, but 95% sure LAX is not one of those.

I expect the Mexico flight will be separate ticket the Vietnam flight, so allow plenty of time. With separate tickets if the first incoming flight is late and you miss the next flight all the risk it with you.

https://www.cbp.gov/travel/advisories-wait-times
https://awt.cbp.gov/

February is seven months away, so I'm not sure why you think you wouldn't be able to get Global Entry between now and then. If you can't find an appointment just keep checking for cancellations.

You shouldn't have an issue getting Global Entry before your trip. Once you have it, you'll really appreciate the time it saves! As said above, cancellations is also another way to secure the in-person interview. You might even want to consider your interview on arrival: https://www.cbp.gov/travel/trusted-t...llment-arrival

Was hoping I wouldn't have to go through customs only to have to go through customs again in Mexico and the again back into the US. Sounds like it may be worth it to get global entry.

You seem to be mixing up Global Entry - expedited immigration processing - with having to go through customs.
Also, a number of US airports are now combining immigration and customs in one stop ... though you are still subject to random stops before exiting the terminal.

Only connecting international to International at DFW and MIA can you through check baggage (you still must process immigration, exit through customs and return airside through security).

You’ll probably arrive at (B) Tom Bradley International Terminal. At LAX you’ll have to process through it all, retrieving your baggage after you clear immigration, processing it through customs and rechecking it in the facility as you exit customs, reprocess through security (probably at your departure terminal). If you’re not on the same PNR, you may well have to lug bags and check in with your next carrier.

At LAX, I’d recommend a three hours to make this relatively stress free, including boarding your next flight. You can reduce some time in immigration by using Automated Passport Control (APC); read here. But you may still require time to wait for your baggage and process through any customs queues.

If you have through ticketing, your connecting carrier will strive to get you off to MEX on the next flight with available seats if you miss your originally scheduled flight. If not, depending on the airline, you could be required to buy another ticket.

I am relatively new to international travel, I have been to Mexico 3-4 times as an adult but didn't really remember the process. I do remember those apc things though, and they were pretty quick. Turns out I have a pretty good layover so it shouldn't be an issue. Now I just have to get all my immunizations for Vietnam which are apparently quite spendy. Thank you guys, some day I will be as informed as you.