It sounds like you've tried a lot of things!
A couple of things that no has mentioned yet - pumping while your LO is nursing on the other breast. It sounds like your not letting down for the pump, and this can help by having your LO do the work for you. This however won't help when you are away, but it might help in the meantime to get some more milk.
The morning is a good time to pump, many mothers find that their supply is higher in the morning. Of course if you aren't letting down for the pump, it's not going to help, but it can't hurt to try!
Pumping at the same time everyday was mentioned before, that's a good idea as you cannot expect to get much with your LO nursing on demand. Getting even a half an oz is good!
I understand that this is a get away trip for you, and depending on your activities you might find that Vegas is a little more family friendly than expected. Of course not at 3 am in a casino, but they do have great shows and restaurants.
A hospital grade pump and hand expressing are both good ways to help that let down occur. Hand expressing is an art for sure, you said you got a little bit of milk (but again no let down?) so this may or may not work for you while you are away - but with a little more practice it might work for you.
I cannot stress (no pun intended) how much being not stressed will make a difference. I know that you said you had tried to relax, pumped in a dark room, that sort of thing. But to try to be relaxed is not usually relaxed KWIM? Being worried that you won't have enough milk for her while your away, or looking down and seeing nothing in the bottle can halt any chance of milk coming for the pump. You could try some deep breathing and relaxation exercises - doing something mindless that you enjoy can also help. I know a mom who had a sudden drop in her pumping output, only to discover that the new tetras game she started playing while pumping was the culprit!
Pumping is an art for sure! I hope you can come to a solution that works for you and your family, good luck!