These are really great questions so after I read them yesterday I wanted to spend some time thinking about them before answering.
I think the answer is that it's no more unethical to take an international flight for a vacation than it is to simply exist as a typical modern human.
If you skip the international vacation due to ethical concerns, then those same ethical concerns obligate you to avoid using any product shipped via air freight. Some of the food you eat may have been shipped internationally by air. The clothes you wear and the smartphone in your pocket almost certainly were, as was the laptop on your desk. Maybe even the desk itself.
As @jpop points out above, there are more effective ways for individuals to reduce their carbon footprint. But I think what we really need are large, systemic changes. We need to make it easy for everyone to reduce their carbon impact. We need better alternatives.
When there's a better alternative to a carbon-heavy activity or resource, people adopt it quickly. For instance, as electric cars become more affordable and more capable, more people are happy to get rid of their gas cars. When people learn that there's a great farmer's market nearby, they're excited to buy locally produced food. As technology improves and costs fall, people are eager to have solar panels installed on their homes.
We don't yet have a good low-carbon alternative for international travel, but with enough effort and investment in other areas we can make changes that will offset the impact of the things we can't change yet.