I submitted my vacation weeks ago. Got approval in the system and from my boss. Everything is fully booked, non refundable. My boss now is saying I have to attend an event on the dates of my vacation and that it’s mandatory. I have respectfully decline twice and he keeps emailing me back stating that it is mandatory and I need to make arrangements to attend. Advice on how to proceed? I’m planning on writing HR or his boss but not sure who to write or how to handle. Thanks.