I wouldn't worry about fitting in either. I've seen plenty of couples dressed very well at restaurants of varying calibers in Las Vegas. If you enjoy dressing up, do it. Don't worry about fitting in.

On the other hand, if you are concerned with the attire of other diners, then Vegas is probably not for you. Last time I cared to look into it, Joel Robuchon and Guy Savoy were the only big-name restaurants left in town that still listed jackets as required on the website but that is by no means a strict requirement (I have eaten at Robuchon without a jacket and I've even seen jeans/polos in the dining room).

Le Cirque and many other restaurants that used to require a jacket have long since given up. Personally, I think it's worse that Robuchon and Guy Savoy claim to require them but don't enforce it.