Would love to hear an update on this. Please tell me she's been wearing thongs! Anyways, here's my thoughts:
Of course she wants to be a thong girl. Every woman wants to be a thong wearer. For some reason though, your GF is a total headcase about it. She's got a mental hangup about thongs. Being that she is so reluctant to wear thongs in her daily life, I'm shocked that's all she packed for her vacation! It's great news for you of course, but I don't understand how she can have the confidence to rock a thong bikini in public yet not feel comfortable wearing thong underwear at home.
I've said this before in the past, but I am convinced the problem is her mental perception of thongs. She considers them to be "slut wear," something that's appropriate to be used when turning YOU on. She does NOT see them as normal, practical, every day underwear appropriate for work. The only way to combat this mindset, well you said it yourself. You have to get her to wear them on a daily basis. It needs to become routine. After wearing thongs for a week straight she'll realize that only two things in the world have changed: (1)she no longer has unprofessional pantylines and (2) you have a constant boner. She needs to realize and accept that thongs are primarily meant to be worn as everyday underwear.
Her self-esteem and body issues could definitely play a part in her avoidance of thongs, but I think it's more her perception that is the problem. I don't think a woman with a body image complex would wear a thong bikini on vacation.
Some might say she just doesn't find thongs physically comfortable, can't stand how they ride up her butt, or whatever. I don't buy that. First off, I wear men's thongs myself so I know how it feels. If you have quality, correctly sized thongs, then you won't even notice the bit between your cheeks. Secondly, when you were on vacation and she was wearing thongs the entire time was she doing any of the following: squirming uncomfortably/walking funny, picking at her butt, or complaining about her wedgie? If you answered "No" than it's proof that she is comfortable wearing thong underwear.
My advice is to keep fighting, don't give up. Keep the fitness routine going, she NEEDS to feel good about her appearance. Also, make sure she knows how beautiful you think she is, ESPECIALLY when she is wearing thongs. Constantly reinforce how thongs are normal every day underwear, and not just fun items that turn you on. If she is getting dressed in the morning and has god-awful panty lines, bring them to her attention. Tell her this is the reason thongs exist. And above all, do whatever you have to do to get her to wear them regularly. It's awesome how you go panty shopping together, but only buy her thongs and g-strings. Never compromise with other styles. You have to start decreasing her supply of fullbutts as well. For your next game/bet, instead of asking her to wear a thong (which she probably won't do anyways), agree to throw out a pair of her granny panties.
I'm just throwing it all down here, obviously you gotta be subtle with all of it. Don't give up, I have faith in her. Even though your GF might be the hardest person to convert to thongs ever. LOL.