UA can and will through check the bag since the 2nd ticket is *A and it’s a short transit. Don’t worry about SQ’s weight limit. But do give SFO agent extra time (and your script). I have done this 2 ticket thing to SQ many times (including summer 2022) and once to AI. Several times with more or heavier luggage than the SQ standalone ticket would allow.

The other direction SGN-SIN//SIN-SFO of course you are at mercy of SQ. I have been successful doing a through check this way too, from India though not at SGN.