I think I maybe solved that problem. In the sparkup.vim file, in the 26th line I changed the
let g:sparkupNextMapping = '' to something else (I changed it to as in forward, but that is also they combo for moving downwards in the document so I have to think of another one). I tried before to change that in the .vimrc file, it didn't work for me, maybe I did it wrong. So after the code is translated in to html you have to use that binding to jump from entry field from entry field instead of the tab key. It isn't the best solution I'm sure, the best way would be to make that keybinding in Sparkup situation aware, but that is well beyond my knoweledge:).

I will try Less.
Well in vim, if you use Sparkup and try to use the tab key two things can happen in my experience. First if you use tab after a valid html trigger for Sparkup (like html, head etc) it will convert it, but only the last tag (it evaluates the whole line only with or with a custom trigger you set). Second if you use it in an empty line or after some normal text it will do nothing or jump back a character, and display a ":call 93_SparkupNext()" message. I'm just telling you this, because I've seen that you started to use vim (and you post a lot of vim related link which I like very much), and maybe you'll have the same problem.

In your opinion which one is better: Sparkup or Zen Coding? Which one do you use?
Has Zen Coding the same annoyance that Sparkup has, namely that you can't use the Tab key to indent? I'm a beginner in both vim and html (it's merely a hobby at this point) so I couldn't fix this particular problem (but I read about this problem on a ticket at Sparkup's gihub repo). Also what's your opinion on Haml/Sass? I know that's a lot of questions, feel free to ignore me:).