You know I have to tell you, it is going to be very difficult not to arouse your girlfriend's suspicion. You two are staying at a hotel in Vegas overlooking this lake, that has a water show you and her both know she loves. Couple that with the fact she knows you have the ring, it is likely she will know what's coming. That being said, you should still do it in Vegas and you should still incorporate the water. I don't know much about the Bellagio (with exception to what I saw in Oceans 11) but maybe you two could go for a nice walk and come back to the fountain and propose to her right in front of it. Or perhaps somehow have her pull the ring out of the water( that would require good planning, security and a durable ring box). I have a feeling if she is anything like myself or the majority of other females out there, just being in Vegas period will make her suspicious (no matter what you tell her) the key is to make the proposal when she least expects it. I hope that helps!
Why don't you keep mentioning another pretend trip or some place you just have to go to with her--and only her. Make this big deal out of something that is going to happen a week or 2 after vegas ---she may think it will be then.