Did you get a message from an ncr trooper saying that the ambassador wants to meet you?

there is no way to mix.  if you keep working with yes man, the ncr will want nothing to do with you. i already tried this. you get a quest called "Dont tread on the bear" which just says, dont keep working with Yes Man or else the ncr will no longer ask you for help

Its kind of bullshit, and is making me like the game just a little less. 

You can string people along for a long time before you have to make a decision.  Just finish all the Side Bets quests (they are identical to the final quests from House and the NCR) and you will end up with "Don't Tread on The Bear".  At that point, you have to decide what you want to do, but you are basically one or two missions from the end, so just save it and finish the game one way, and if you don't like it, load it up again and finish it another way.

You can put yourself about an hour away from the House, NCR, or Yes Man ending and make a hard save, then go burn some bridges. Idunno if there's a brotherhood ending. Its so easy to just reprogram the turrets and the bunk is basically 70% dead. Why bother helping them, you know. I have a feeling they just become a faction that supports NCR instead of having their own ending.

" You can put yourself about an hour away from the House, NCR, or Yes Man ending and make a hard save, then go burn some bridges. Idunno if there's a brotherhood ending. Its so easy to just reprogram the turrets and the bunk is basically 70% dead. Why bother helping them, you know. I have a feeling they just become a faction that supports NCR instead of having their own ending. "