I did the VS LHR-LAX & UA LAX-HNL thing too some 18 months ago routing back via SFO. You need to be aware that UA don't really communicate with VS, so say you have to make a connection without a stop over at SFO you can not check yourself in for LHR leg until you arrive at SFO (though you can route your bags straight through!) & if you are running late inbound, you may incur problems, desks way apart......... I did ! Also if you route via LAX outbound without a stopover you will have a major terminal change which can not be walked, and you will also have to collect & clear bags at LAX. Again you will have to recheck yourself in & suffer waiting another line. It can all really be a pain in the backside. As for UA equipment, they were flying 767's and seats in front ecconomy sections had much better seat pitch than those in rear. It seemed to me these were allocated on a random basis and were infinitely better should flight be full, and some attempt at a rest was required !
All in all the Hawiian Islands are a great place & worth the effort.