I'd suggest paying more attention to how the code shows up in Github next time (assuming they're reviewing it on Github). If I'm reviewing a potential hire's code, I'd want to see that they pay attention to details like that, lest the production code base get super ugly / hard to read.

Yeah, I confess I'd find it hard to review someone's code with random-looking indentation going on, it's easy to imagine someone treating it as a bad sign. Definitely fix this for your next interview, keep trying though. :-)

listchars works pretty well. I have this in my .vimrc:

    set list
    set listchars=tab:>=,trail:~,extends:>,precedes:<,nbsp:.

Edit: you can also configure it to use one type of indentation. I use "set expandtab" so that my files are always indented with spaces.