This is definitely a tough one - I'm having my own difficulties with the BM situation (see another thread).
I'm sure we could all sit her and psychoanalyze - maybe she is jealous? Maybe she is mad about not being MOH? Maybe she has stuff going on at home?
But at the end of the day, she isn't supporting you the way you need to be supported.
If it were me, I would likely handle it by getting her together for coffee and asking if she is ok in general. Is something going on you don't know about? At the end of the day, you are still friends.
But when it comes to the wedding - I would probably tell her that yes, you are at different places in your life. And if she feels that it is best not to stand up with you for that reason (or others), then she can step down but you won't replace her because she isn't replaceable in your life. I would seriously give one groomsmen two bridesmaids to walk with to account for an uneven number. And with all that, let her know that you would still really like to share that day and week with her - maybe it will help you two to reconnect.
Then she has no pressure to book the vacation. And if things improve before your wedding (I think its early 2016??) you can offer her back the spot. She knows she will have to pay more.
And if it is really that important to YOU, maybe secretly put down the $150 for her to hold that seat as a just in case. All you lose is $150 - and you could even get someone else who has booked yet to take that spot if necessary.