Agree with above, no good options for connecting LAS to SFO.

>> If you really want to do CX/OW all the way you will need to do a connection through PHX (LAS - PHX - SFO) so the routing will be SIN - HKG - LAX - LAS - PHX - SFO - HKG - SIN (don't know about your company policy or travel budget but doing this as a multi-city / stopover is pricey on CX J)
>> An alternative would be to book SIN - SFO - SIN round trip on CX, then book a separate ticket SFO - LAS - SFO on UA (you will need to leave sufficient buffer on the outbound to SFO so you don't misconnect since this won't be a protected transfer)
>> Last alternative for fastest routing will be on SQ (SIN-SFO-LAS-SFO-SIN). [I take this route probably once a year because I visit family who live both in the bay area and in Vegas, albeit having an additional MNL - SIN sector]