Flying 2 separate itineraries. LAX-PEK-HKG Air China, then HKG- SGN Vietnam Airlines
 

4 hour layover HKG

At LAX, Can Air China have my bags checked all the way thru to SGN?

Do Air China (Star Alliance) and Vietnam Airlines (Sky Team) have any type of inter-airline agreement?

If not, will i have to completely clear customs/immigration in Hong Kong, go to street level, return to the airport ticket counter and clear immigration again with HKG officials?

I think both Air China and Vietnam Airlines are in Terminal 1, will that make a difference?

Finally, I will have a wheelchair bound parent with me. Will this make it more difficult to transit thru ?

No. This is a very bad idea. Do not put your disabled parent through this.

PEK/PVG -HKG flights are some of the most frequently delayed in the world and you are very likely to miss your flight in HKG .VN will have no mercy and I doubt CA will check your bags through anyway,

Please re-assess your plans...

Agree this is a problematic itinerary, particularly with the PEK connection.

Understood.

However, itineray has been purchased.

Will Air China be able to check my bags all the way thru?

If not, do i have to go thru both immigration AND customs, get to my bags, re-enter airport for the Vietnam Airways flight?

Because the tickets were purchased separately, merely having a interline agreement does not mean CA will be willing to check a bag thru in this case. So you'd need to clear immigration, pick up bags, and check in. It looks like you will be flying an early morning flight to HKG, those have tend to be more reliable so you should be okay. Although connecting in PEK early morning can be stressful due to the long line ups.

I would definitely not expect CA to check through, although I suppose it's possible.

It has been reported that CX at HKG will pull your bags off the carousel and tag them to your final destination so you don't have to clear immigration. I don't know that VN will do that, but it might be worth stopping by the VN transit desk and asking if it's possible. If not, the whole process of clearing immigration inbound, retrieving your bags, rechecking them, and clearing immigration outbound will probably take 60-90 minutes.

With the 4 hour layover, you should be fine.