To be honest, when I made Vimdeck, it was mostly just-for-fun. I never really expected that other people would seriously consider using it for their presentations.
Now that I've seen how many people are interested in it, though, I might spend some time adding more features to it to make it a more viable option.
As you pointed out, there's not a great way to take this format and use it on someone else's machine.
First, I'd like to point out that all Vimdeck is doing is generating files into a directory and opening them with VIM. There are a few VIM plugins as dependencies. Like markdown syntax highlighting, and SyntaxRange, but it's entirely possible that I could allow the users to bundle their presentation as a zip file, put it on a flash drive and open it on any other computer (most have VIM already installed).
There's no Gvim necessary. Just a couple of plugins and the dynamically generated VIM script. Seems plausible to be able to bundle all that into a machine-agnostic format. VIM already runs everywhere.
And as far as the font size goes, you're right. There's no way around it. You're going to have to increase/decrease the font size immediately before the presentation to get it just right. But there's often a chance for speakers to test that their slides work in the current environment any ways.
I'm not saying text-only ASCII-art-ridden presentations are the best way to present information. But depending on the crowd and the speaker, it might be worth it.