Given free rein, I’d absolutely love to tell people that needing me to show them how to do X in Excel is actually not a vacation-interrupting emergency and there are tons of free videos that would explain that, if they did not want to contact the actual departments who handle tech support and training. Or that this project they’ve known about for a month but decided to keep under their hat until it became an emergency is something they’ll need to resolve themselves. But that would not fly at all.
After all, your goal is always to keep your business associates happy. For that reason, you should have a backup co-worker who will cover some of your work while you are not available. Not only is this the professional way to go, but it will also leave your customer satisfied with the company’s service.

We’ve all been there. A balmy evening beckons and across the street a crowd is already spilling from the pub, fanning out across the pavement in summer dresses and rolled-up shirt sleeves. But as you frantically try to clear your desk for the weekend, every email you send prompts a suspiciously swift reply. Yes, it’s the dreaded out-of-office auto-response, set to tauntingly remind you of a world of leisure while simultaneously pushing it further from your reach.
Hi, I am currently out of the office until [MM/DD]. I will reply to emails as soon as I can upon my return. If this is urgent, please contact [name] at [email] or [phone]. In the meantime, check out this new [product/sale/service/etc]. I can answer any questions about it for you when I get back.