Bring proof of your travel arrangements with you to the interview. If you are approved and the time frame is too tight to ensure that the green card will arrive in time they may stamp your passport with the I-551 stamp. Or - they may tell you to wait for the green card, but you can make an infopass appointment for closer to when you are to leave and if you have not received your green card, again request the stamp as you have imminent travel plans. If you are not approved at the interview, then the AP will still be valid until there is a decision. If you are out of the country when the decision is made you should still be able to return on the AP explaining that you have not been back since the green card was approved so could not get the actual card into your possession. If it looks like the green card won't be approved at all, then don't travel until you do have a decision. (Not likely to happen, but you know your circumstances best).
