You'll have to book this through Expedia (multicity option). AA.com doesn't let you do things like this... Best fare right now is ~$120 and you'll need to spend the night in SAN. When Southwest has a fare sale the price will drop.
SJC-LAS
11:20 AM - 12:47 PM
American Airlines 1988
LAS - LAX - SAN
1:35 PM - 4:24 PM
American Airlines 1059 / 3227
SAN - LAS - LAX
3:30 PM - 6:23 PM
American Airlines 3310 / 1236
LAS - SJC
7:10 PM - 8:30 PM
American Airlines 1705