This is really old news, as I passed the baton a while back, but I never did get around to posting a note about it. I have passed maintenance of Usermin::Vacation off to Jamie Cameron, the author of Webmin. Thank you, Jamie!
I haven’t really had a need or desire to continue work on the module over the last year or so. I know a bunch of folks have been looking for alias table support, but I haven’t had the inspiration to put it together. Perhaps if you ask Jamie nicely, he’ll work it in. He knows it’s a requested feature.