I don't have specific information for vanuatu, but I will address two things you are asking from a more general standpoint:
1) Most PCVs have primary assignments and secondary assignments. The primary assignment is the one you get sent over there to do and the secondary assignments are the ones that you do because you have an interest or see a need in your host location. As long as you are still serving your host organization, and working with them regularly, most posts are very liberal about how much time and energy you dedicate to secondary projects. If HIV awareness is something you are interested in doing, then you will ABSOLUTELY be able to do something. However, I am not sure if they have transitioned to the new system where you are invited directly to an organization. if they have not, then sometime during training your in-country staff will place you with an organization. Make sure they know from the beginning that you have the desire to work on HIV issues as a secondary assignment and this may affect your placement. (They really want their volunteers to be happy because engaged, happy volunteers = a higher retention rate)
2) Drinking in Bulgaria is very popular and it can be hard to refuse a drink. However, I found that it was more difficult for me to refuse (because I wanted to drink) than for my Bulgarian counterparts to accept that I didn't want to drink. Your hosts know you are from a different culture, and may be more respectful of your choice than you think. If nothing else works, you can always say that peace corps has forbid you from drinking.